The results of skin graft exchanges between members of a partially inbred line of White Leghorns provide evidence that the C blood group locus is also a histocompatibility locus or is closely linked to such a locus. The effects of two C alleles were investigated. One allele showed marked influence on graft rejection while the effects of the second allele were not differentiated from the effects of other histocompatibility loci segregating in the population. Previous studies have shown that C antisera have lymphocyte agglutination properties.