Apparatuses, methods, and systems are disclosed for using a medium access control protocol data unit in a message 3 buffer. One method includes, in response to a medium access control protocol data unit being in a message 3 buffer and receiving an uplink grant: obtaining the medium access control protocol data unit from the message 3 buffer; if a first size of the uplink grant does not match a second size of the medium access control protocol data unit: indicating to a multiplexing and assembly entity to include medium access control sub-protocol data units carrying medium access control service data units from the obtained medium access control protocol data unit in a subsequent uplink transmission; and obtaining the medium access control protocol data unit from the multiplexing and assembly entity.