Normal human granulocytes and lymphocytes were preincubated in 0.5, 5, or 50 mg/l of clindamycin, and 5, 50, or 500 mg/l of cefuroxime. Incubation with clindamycin caused an increase in the proportion of granulocytes bearing receptors for the Fc portion of IgG (Fc gamma-R) and C3b (C3b-R). Random migration in capillary tubes was significantly decreased, but phagocytosis as measured by chemiluminescence was only slightly decreased. The proportion of lymphocytes bearing Fc gamma-R was decreased, while there was no effect on lymphocyte C3b-R percentage, nor on the proportion of granulocytes or lymphocytes bearing sheep erythrocyte (E) receptors. Preincubation of granulocytes in cefuroxime was not associated with changes in the proportion of receptor-bearing cells, except for a slight increase in Fc gamma-R-bearing lymphocytes at the lowest concentration tested. Tube migration was not affected but chemiluminescence was significantly decreased. Preincubation with clindamycin, although increasing the proportion of cells bearing phagocytosis-associated receptors, is thus not associated with an increased phagocytic ability, while cefuroxime incubation caused a decrease in chemiluminescence despite normal levels of receptor-positive cells.