diff --git a/CHANGELOG.md b/CHANGELOG.md
index a3508b009..151a3761f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,18 @@ A breaking change will get clearly marked in this log.
 
 ## Unreleased
 
+### Added
+- `contract.AssembledTransaction` now has a `toXDR` and `fromXDR` method for serializing the
+transaction to and from XDR. Additionally, `contract.Client` now has a `txFromXDR`. These methods
+should be used in place of `AssembledTransaction.toJSON`, `AssembledTransaction.fromJSON`, and
+`Client.txFromJSON` for multi-auth signing. The JSON methods are now deprecated. **Note you must now
+call `simulate` on the transaction before the final `signAndSend` call after all required signatures
+are gathered when using the XDR methods.
+
+### Deprecated
+- In `contract.AssembledTransaction`, `toJSON` and `fromJSON` should be replaced with `toXDR` and
+`fromXDR`. Similarly, in `contract.Client`, `txFromJSON` should be replaced with `txFromXDR`.
+
 
 ## [v12.0.1](https://github.com/stellar/js-stellar-sdk/compare/v11.3.0...v12.0.1)
 
diff --git a/src/contract/assembled_transaction.ts b/src/contract/assembled_transaction.ts
index 1f46824b9..de584fb81 100644
--- a/src/contract/assembled_transaction.ts
+++ b/src/contract/assembled_transaction.ts
@@ -28,6 +28,7 @@ import {
   implementsToString,
 } from "./utils";
 import { SentTransaction } from "./sent_transaction";
+import { Spec } from "./spec";
 
 export const NULL_ACCOUNT =
   "GAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAWHF";
@@ -364,6 +365,47 @@ export class AssembledTransaction<T> {
     return txn;
   }
 
+  /**
+   * Serialize the AssembledTransaction to a base64-encoded XDR string.
+   */
+  toXDR(): string {
+    if(!this.built) throw new Error(
+        "Transaction has not yet been simulated; " +
+        "call `AssembledTransaction.simulate` first.",
+      );
+    return this.built?.toEnvelope().toXDR('base64');
+  }
+
+  /**
+   * Deserialize the AssembledTransaction from a base64-encoded XDR string.
+   */
+  static fromXDR<T>(
+    options: Omit<AssembledTransactionOptions<T>, "args" | "method" | "parseResultXdr">,
+    encodedXDR: string,
+    spec: Spec
+  ): AssembledTransaction<T> {
+    const envelope = xdr.TransactionEnvelope.fromXDR(encodedXDR, "base64");
+    const built = TransactionBuilder.fromXDR(envelope, options.networkPassphrase) as Tx;
+    const operation = built.operations[0] as Operation.InvokeHostFunction;
+    if (!operation?.func?.value || typeof operation.func.value !== 'function') {
+      throw new Error("Could not extract the method from the transaction envelope.");
+    }
+    const invokeContractArgs = operation.func.value() as xdr.InvokeContractArgs;
+    if (!invokeContractArgs?.functionName) {
+      throw new Error("Could not extract the method name from the transaction envelope.");
+    }
+    const method = invokeContractArgs.functionName().toString('utf-8');
+    const txn = new AssembledTransaction(
+      { ...options, 
+        method,
+        parseResultXdr: (result: xdr.ScVal) =>
+          spec.funcResToNative(method, result),
+      }
+     );
+    txn.built = built;
+    return txn;
+  }
+
   private constructor(public options: AssembledTransactionOptions<T>) {
     this.options.simulate = this.options.simulate ?? true;
     this.server = new Server(this.options.rpcUrl, {
@@ -412,14 +454,16 @@ export class AssembledTransaction<T> {
   }
 
   simulate = async (): Promise<this> => {
-    if (!this.raw) {
-      throw new Error(
-        "Transaction has not yet been assembled; " +
-        "call `AssembledTransaction.build` first.",
-      );
-    }
+    if (!this.built) {
+      if (!this.raw) {
+        throw new Error(
+          "Transaction has not yet been assembled; " +
+          "call `AssembledTransaction.build` first.",
+        );
+      }
 
-    this.built = this.raw.build();
+      this.built = this.raw.build();
+    }
     this.simulation = await this.server.simulateTransaction(this.built);
 
     if (Api.isSimulationSuccess(this.simulation)) {
diff --git a/src/contract/client.ts b/src/contract/client.ts
index e0c298b4a..f576762a6 100644
--- a/src/contract/client.ts
+++ b/src/contract/client.ts
@@ -124,5 +124,8 @@ export class Client {
       tx,
     );
   };
+
+  txFromXDR = <T>(xdrBase64: string): AssembledTransaction<T> => AssembledTransaction.fromXDR(this.options, xdrBase64, this.spec);
+
 }
 
diff --git a/test/e2e/src/test-swap.js b/test/e2e/src/test-swap.js
index 129c181be..71e0e0ab4 100644
--- a/test/e2e/src/test-swap.js
+++ b/test/e2e/src/test-swap.js
@@ -100,31 +100,32 @@ describe("Swap Contract Tests", function () {
     expect(needsNonInvokerSigningBy.indexOf(this.context.bob.publicKey())).to.equal(1, "needsNonInvokerSigningBy does not have bob's public key!");
 
     // root serializes & sends to alice
-    const jsonFromRoot = tx.toJSON();
+    const xdrFromRoot = tx.toXDR();
     const { client: clientAlice } = await clientFor("swap", {
       keypair: this.context.alice,
       contractId: this.context.swapId,
     });
-    const txAlice = clientAlice.txFromJSON(jsonFromRoot);
+    const txAlice = clientAlice.txFromXDR(xdrFromRoot);
     await txAlice.signAuthEntries();
 
     // alice serializes & sends to bob
-    const jsonFromAlice = txAlice.toJSON();
+    const xdrFromAlice = txAlice.toXDR();
     const { client: clientBob } = await clientFor("swap", {
       keypair: this.context.bob,
       contractId: this.context.swapId,
     });
-    const txBob = clientBob.txFromJSON(jsonFromAlice);
+    const txBob = clientBob.txFromXDR(xdrFromAlice);
     await txBob.signAuthEntries();
 
     // bob serializes & sends back to root
-    const jsonFromBob = txBob.toJSON();
+    const xdrFromBob = txBob.toXDR();
     const { client: clientRoot } = await clientFor("swap", {
       keypair: this.context.root,
       contractId: this.context.swapId,
     });
-    const txRoot = clientRoot.txFromJSON(jsonFromBob);
+    const txRoot = clientRoot.txFromXDR(xdrFromBob);
 
+    await txRoot.simulate();
     const result = await txRoot.signAndSend();
 
     expect(result).to.have.property('sendTransactionResponse');