SummaryTests of over 2000 pigs, with the blood group reagents Ac, Ap and 0, and the reagents Ha, Hb, He, Hd, He and Hc/c, confirmed that theHlocus affects the expression of A and 0 antigens. The ‘exceptional’ phenotypes Ap and — were found in Ha homozygotes of Miniature pigs and in Hcd homozygotes of a semi‐inbred Landrace line. They were also found in unrelated pigs of other breeds, and moreover in some individuals with genotypesHaHcd,HabHcd,H‐H‐andHaHaborHabHab.The mechanism of the effect of theHlocus on the A phenotypes may consist in the interaction of certain, not yet completely defined,Hgenotypes, or in linkage of the H system with the hypothetica