SummaryThe investigation of A‐O blood group phenotypes in selected pig families confirms the existence of anSlocus which specifically controls the expression of A and) alloantigens. Forty‐five informative litters were scored for linkage between theSgene and theHblood group locus. The recombination frequency in 345 offspring was estimated to be 9.56 % (33 cross‐overs). Specific difficulties involved in the determination of A‐O and H phenotypes and the importance of S and H polymorphisms for the determination ofHalgenotypes are di