Studies concerning the activity of cefodizime (HR 221), on certain aspects of the immune response, were conducted. It was found that lymphocytes from Balb/c mice treated with 3 and 30 mg/kg/day of cefodizime display increased responsiveness to B-cell mitogens and specific antigens. Also, the amount of antigen specific antibody producing plaque forming cells was increased in these mice and was accompanied by a rise in the specific IgG haemagglutinin titer. These effects were not observed in lymphocytes obtained from NMRI mice that had been treated with cefodizime. Peritoneal macrophages from NMRI mice, treated with cefodizime prior to harvesting of the cells, contained increased levels of lysosomal enzymes, developed enhanced chemiluminescent reaction to stimuli and showed elevated pinocytosis rates. Furthermore, NMRI mice treated with cefodizime during the immunization, developed enhanced DTH-reaction, when challenged with the antigen (SRBC). The prophylactic treatment of Balb/c mice with cefodizime (2×30 mg/kg/day ip for 4 days) significantly prolonged the mean survival time of the animals after intravenous infection with Candida albicans 200/175 (16.7 days as against 3.5 days in the case of the controls). This stimulatory effect of cefodizime on the host defence system was not observed for NMRI mice. Treatment with latamoxef or cefoperazone under the same experimental conditions did not reduce the susceptibility of mice to C. albicans. The protective activity of cefodizime against C. albicans in Balb/c mice, may be due to the immuno-stimulatory activity of this agent.