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ABO phenotype frequencies in 2056 blood donors from the period 1946-1950 were compared with those found in the live donor panel of 1971 (n = 1657). In this 21 year interval the ABO distributions had changed, P < 0.02, there being a decrease in group A and a compensatory increase in B. The incidence of O phenotype remained constant. It is suggested that the most plausible explanation of this change is the marked immigration to the Island since 1950 principally from other parts of the British Isles. Additionally, the 1950 data, subdivided for increasing Manxness employing surname analysis, were compared with an indigenous sample of 1971 but no differences in the proportions of ABO groups were detected. However, significantly different ABO frequencies occurred among donor and non-donor samples in this 1971 series and possible explanations were discussed.