Infections in immunocompromised patients are often difficult to treat, even with currently available antimicrobial agents. An understanding of the effects of antibiotic therapy on the host's immune response is therefore important when deciding on the clinical management of such patients. Antimicrobial agents which lack immunodepressive effects and/or potentiate the immune response are the goal of current research into the treatment of infections in immunocompromised patients. The effects of cefodizime (1-250 micrograms/mL) in vitro on some functional activities of leucocytes and on colony formation by granulocyte monocyte progenitors were studied to investigate the effects of the antibiotic on the host's immune response. A marked enhancement in the lymphocyte transformation reaction was observed in cells exposed to cefodizime. This effect was dose-dependent. Cefodizime had no significant effect on antibody-dependent cell cytotoxicity or on natural killer cell-mediated cytotoxicity. The chemotactic activity of neutrophils was not influenced by the presence of cefodizime (P > 0.05). The phagocytic activity of neutrophils was significantly increased by cefodizime (P > 0.01). Cefodizime significantly stimulated, in a dose-dependent manner, colony formation by granulocyte-monocyte progenitors (P < 0.01). Results suggest that cefodizime has certain stimulatory effects on immunocompetent cells such as enhancing the transformation reaction of lymphocytes, neutrophil phagocytosis and colony formation by granulocyte-monocyte progenitors. Further studies are required to clarify the mechanisms responsible for these effects.