Optimal conditions for obtaining the proliferative response in mixed kidney cell‐lymphocyte culture (MKLC) have been determined in rats. The addition of 2‐mercaptoethanol to the culture medium was essential for successful stimulation of lymphocytes by allogeneic kidney cells. Proliferative responses by standard MLC and MKLC were compared in rats with the same or a different Ag‐B locus. In most strain combinations, there was a significant correlation between MLC and MKLC reactions. However, the DA‐ACI combination, compatible at the Ag‐B locus, consistently showed negative MLC and positive MKLC reactions. This suggested the possibility that this technique allowed the expression of kidney specific antigens.