SummaryPigs of the Hampshire and Duroc breeds were subjected to bloodtyping. An anomaly of inheritance affecting the E blood‐group system was detected in one pig among 10056 progeny from 1171 matings. AnE1E3Hampshire male was mated with anE1E1Hampshire female. The male had been exposed to 300 r of gonadal X‐irradiation. Thirteen pigs were born. Six pigs wereE1E1, six wereE1E3, and one appeared to beE1E2.The exceptional bloodtype might be a result of mutation, interallelic recombination or gene interaction. These hypotheses are discus