To investigate the therapeutic potential of killed Corynebacterium parvum, its effects on the course of Listeria monocytogenes infection in mice was studied. Mortality in mice given C. parvum after L. monocytogenes infection was greater than in mice given C. parvum before infection or infected with only L. monocytogenes. C. parvum alone resulted in no mortality. Spleens from infected mice given C. parvum had increased numbers of L. monocytogenes. Peritoneal macrophages from mice infected with only L. monocytogenes were activated by four days after infection, whereas those from mice given L. monocytogenes plus C. parvum were not. By seven days macrophages from both were activated. Delayed hypersensitivity and in vitro lymphocyte transformation in response to L. monocytogenes antigen were strikingly suppressed in infected mice given C. parvum. The increased susceptibility of L. monocytogenes-infected mice given C. parvum may have been due to delayed macrophage activation resulting from suppression of thymus-derived lymphocyte responses.