The present invention provides a soluble T cell receptor (sTCR), which comprises (i) all for part of a TCR .alpha. chain, except the transmembrane domain thereof, and (ii) all or part of a TCR .beta. chain, except the transmembrane domain thereof. (i) and (ii) each comprise a functional variable domain and at least a part of the constant domain of the TCR chain, and are linked by a disulphide bond between constant domain residues which is not present in native TCR.