Serum IgM levels in sickle cell patients in the steady asymptomatic state were determined using radial immunodiffusion and found to be significantly higher than in health controls. Other immunoglobulins (IgA and IgG) were not significantly different from those of the controls. However concentrations of both C3 and C4 components of complements were significantly lower in sickle cell disease. This explains the low opsonization and chemotactic functions observed in these patients and further confirms the transient activation of the alternate pathway of complement. Abnormal complement C3 metabolism may explain the high susceptibility of patients to infections which contributes to the crisis in sickle cell disease.