Patients with sickle cell disease have an increased tendency to develop frequent and severe pyogenic infections. Lymphocyte subpopulations were studied in 14 patients with homozygous sickle cell anaemia (SCA) using the OKT monoclonal antibody. The mean absolute lymphocyte counts observed in patients with SCA were similar to that of controls. The OKT8+ lymphocytes were significantly higher in SCA patients than in controls. The mean OKT3+ and OKT4+ lymphocytes were significantly lower in SCA patients than in controls, and there is a reversal of the normal OKT4+: OKT8+ ratio. The number of B cells present, as detected by surface membrane immunoglobulin, was significantly higher in SCA patients when compared with control values. These preliminary findings suggest an important mechanism to further explain the increased tendency to infection in SCA patients, in addition to the previously described immunological disorders and the asplenic state.