The effects of 7 days' chemotherapy on the humoral and cellular parameters of the host immune system are described. In Balb/c mice the effects of cefotaxime, amikacin, mezlocillin, piperacillin and clindamycin were examined. The delayed-type hypersensitivity reaction, as well as the IgM and IgG responses, were suppressed by four of the five drugs tested: cefotaxime, amikacin, mezlocillin and piperacillin. One to two weeks after completion of chemotherapy with cefotaxime and amikacin, these parameters returned to normal values, whereas the mezlocillin- or piperacillin-modified reactions were still suppressed after 20 days. The concanavalin A and lipopolysaccharide-induced proliferative activities of mouse spleen cells were suppressed, especially by mezlocillin. The possible consequences of the immunomodulating effects of antibiotics for antimicrobial chemotherapy are discussed.