Using the degenerate oligonucleotide primers, we amplified the partial sequence from the pig ABO gene. We later used the sequence information to clone the pig ABO gene cDNA. The pig gene encoded a functional enzyme with A transferase activity. Porcine A and O phenotypes were already known. Therefore, we determined the A/O phenotype of the salivary gland tissues of pigs, and isolated genomic DNA from the pig tissues exhibiting A and O phenotypes. DNA was cleaved with restriction enzymes, electrophoresed, and Southern transferred onto a nylon membrane. After fixation, the filter was hybridized with the radio-labeled pig A transferase gene fragment. The results of hybridization demonstrated that genomic DNA derived from the pig with the O phenotype lacked the homologous sequence. This is in contrast to the human O alleles, which either possess a single nucleotide deletion that causes a frameshift or amino acid substitutions that nullify the enzymatic activity. Apparently, the pig O allele seems to be lacking most, if not all, of the structural gene encoding the glycosyltransferase.