The proliferative response of allogeneic lymphocytes cultured together, the mixed lymphocyte reaction (MLR),2 has been widely used as an in vitro correlate of the antigen recognition phase of the homograft response. Recent studies in mouse and man suggest that the structures responsible for MLR activation are largely coded for by genes which are within the major histocompatibility complex (MHC) of the species (1, 2), but are distinct from the genes controlling the serologically defined major histocompatibility determinants (3, 4). However, the nature and cellular distribution of the gene products which actually mediate recognition of one cell (the target cell) by another cell (the responder cell) is not well understood. The “recognizing” structure is likely to be a specific receptor present on thymus derived (T) lymphocytes since T cells are generally required as responder cells in the MLR (5–7).