Birds of the partially inbred G-B1 chicken line can be classified as either high or low responders to Con A, on the basis of the amount of 3H-thymidine incorporated by Con A-containing cultures of their peripheral blood leukocytes. The pattern of inheritance of the high and low responder traits suggests that the variation in response is due to genetic polymorphism at a single autosomal locus. However, the allele responsible for the low responder trait of the G-B1 line is not identical to the allele of the previously described Mr1 locus carried by the inbred low responder CC line, since (CC x G-B1 low responder)F1 birds are uniformly high responders to Con A. The responses to Con A of mixtures of CC and G-B1 low responder cells are not significantly higher than the responses of either component of the mixture alone. Thus, the gene products responsible for the complementation observed in F1 hybrids cannot complement extracellularly, and are not readily transferable from one cell to another.