Serum IgE concentrations were measured using a radio-radial immunodiffusion method in 4,440 blood donors resident in the Western Cape Province, South Africa, and the following differences were observed. Firstly, in all three ethnic groups studied, that is the Whites, the Cape Coloureds and the Africans, there was a tendency for males to have higher serum IgE concentrations than females. This difference was highly significant in Whites. Secondly, in the Whites, serum IgE concentrations tended to be lower with advancing age. Thirdly, previous reports of elevated serum IgE concentration in indigenous African populations were confirmed. No clearly defined cause for these differences emerged from this study.