The concentration of circulating T, B, and 'null' lymphocytes was determined in thirty children and three adults with Plasmodium falciparum infections in West Africa. During infection, both percentage as well as concentration of T cells were decreased as compared to levels following treatment. The percentage but not concentration of B cells was increased. Both percentage and concentration of 'null' cells were increased in malaria. Patients with splenomegaly had the most severe alterations in T-cell number; no other historic or clinical parameter correlated with the degree or pattern or change in circulating lymphocyte subpopulations. These alterations were rapidly reversible after antimalarial treatment and presumably represent the sequestration of T cells in the spleen or other organs.