The effects on the mitogenic response of human T lymphocytes were studied for 20 different antibiotics. No apparent inhibitory effect could be detected for penicillins, cephalosporins, aminoglycosides, chloramphenicol, sulfamethoxazole, trimethoprim, nalidixic acid, and 5-fluorocytosine. There were effects at high concentrations with erythromycin, clindamycin, and rifampin, and these antibiotics could also be shown to depress the mitogenic response of B lymphocytes. With fusidic acid, nitrofurantoin, and doxycycline there was an inhibiting effect at low concentrations on the mitogenic responses of B and T lymphocytes and on in vitro antibody production. Protein synthesis in unstimulated lymphocytes was also inhibited. Some antibiotics thus may impair the function of human lymphocytes in vitro.