The Am I variant of porcine serum amylase was detected only in pigs having the red-cell antigen I(b), whereas several I(b) positive pigs were found without Am I. Family and population studies show that the chromosomal loci, I and Am, are identical or closely linked. The phenotype of one exceptional progeny from a double backcross mating possibly was a result of paternal irradiation or genetic recombination during oögenesis.