diff --git a/grpc/gcp/handshaker.proto b/grpc/gcp/handshaker.proto
index b1dcd314..ad33eda8 100644
--- a/grpc/gcp/handshaker.proto
+++ b/grpc/gcp/handshaker.proto
@@ -108,6 +108,12 @@ message StartClientHandshakeReq {
 
   // (Optional) Maximum frame size supported by the client.
   uint32 max_frame_size = 10;
+
+  // (Optional) An access token created by the caller only intended for use in
+  // ALTS connections. The access token that should be used to authenticate to
+  // the peer. The access token MUST be strongly bound to the ALTS credentials
+  // used to establish the connection that the token is sent over.
+  string access_token = 11;
 }
 
 message ServerHandshakeParameters {
@@ -118,6 +124,12 @@ message ServerHandshakeParameters {
   // (Optional) A list of local identities supported by the server, if
   // specified. Otherwise, the handshaker chooses a default local identity.
   repeated Identity local_identities = 2;
+
+  // (Optional) An access token created by the caller only intended for use in
+  // ALTS connections. The access token that should be used to authenticate to
+  // the peer. The access token MUST be strongly bound to the ALTS credentials
+  // used to establish the connection that the token is sent over.
+  optional string access_token = 3;
 }
 
 message StartServerHandshakeReq {