A depression in antibody-mediated immunity (AMI) measured both in terms of circulating antibody and plaque-forming cells in the spleen was observed in CFW mice infected with M. lepraemurium when sheep red blood cells (SRBC) and human gammaglobulin (HGG) were used as antigens. The impairment in AMI was evident only after 75 days of infection thereafter the antibody response to SRBC antigen progressively decreased until the last day of experimentation (135 days). Within the first 60 days of infection no alteration in AMI was observed with the HGG antigen while the response to the SRBC antigen was significantly higher in the infected animals than in uninfected controls.