Six antigens detectable by the complement-dependent lymphocytotoxic technique were determined in pigs by six groups of alloimmune sera. It was confirmed that these specificities are controlled by the main histocompatibility region (SL-A). Serological and genetic studies showed that the given specificities (provisionally designated L1 to L6) form at least 6 haplotypes. In addition, family studies confirmed the linkaged between SL-A region and C blood group locus. Maximum lod score values are in recombination fraction omicron = 0.2.