A new red cell antigen, Hc, has been detected in pigs of the Duroc and Hampshire breeds. In the Durocs the 2 iso-hemolysins anti-Ha and anti-Hc are reciprocally related indicating a closed blood group system. The corresponding antigens Ha and Hc are determined by the alleles Ha and Hc. In the Hampshires a recessive allele, H-, is present in addition to the Ha and Hc alleles, thus indicating an open blood group system in this breed. The allele frequencies are estimated and the significance of the findings is discussed.