Previous reports have suggested that a defect in serum complement may contribute to the increased susceptibility to infection shown by patients with sickle cell anaemia (SCA). In order to define the nature of any complement abnormality in SCA, we investigated the complement system in eighty-seven patients during asymptomatic periods, and analysed factor B turnover in a small sample. In these patients geometric mean serum concentrations of functionally active factor B and factor D, and of C3 and C4 protein (expressed as a percentage of normal reference serum) wer lower than in controls (78% vs 107%, P less than 0.001, 86% vs 103%, P less than 0.001, 91% vs 100%, P less than 0.01, 89% vs 105%, P less than 0.05 respectively). The ratio of the serum concentration of functionally active factor B to factor B protein was lower in patients than in controls (means 75% s.d. 16% vs mean 93%, s.d. 22% P less than 0.001), indicating a functional deficiency of factor B protein. In addition, the fractional catabolic rate of radiolabelled factor B was markedly increased in four out of seven asymptomatic patients studied, and was inversely related to the functional factor B concentration in serum (r = -0.59, P less than 0.05); factor B synthesis was uniformly increased. Complement activation was not related to the presence of circulating C1q binding material. We conclude that complement activation, rather than defective synthesis as previously suggested, contributes to the abnormalities in complement componenet concentration and function in asymptomatic subjects with sickle cell anaemia.