Three lines of epithelial-like cells have been developed from the renal cortices of pigs of known histocompatibility type; the cells have been shown to stimulate the development of cytotoxicity in allogenic lymphocytes cultured in contact with them. For this stimulation to occur, it was necessary for the lymphocytes to be of a type that would show blastogenesis in mixed culture with those from the pig used as the source of the renal tissue; serological differences between stimulator and responder cells, as demonstrated by the use of the cytotoxic alloantisera at present available, did not appear to be required.