The purpose of this investigation was to determine the role of the humoral immune response in the production of arthritis in mice immunized with the chlamydial agent of mouse pneumonitis (MoPn) (Chlamydia trachomatis biovar). Mice were made B cell deficient (BCD) by treatment with rabbit antiserum to murine IgM. Control mice included animals treated similarly with normal rabbit serum or phosphate-buffered saline. Male mice were immunized with MoPn inactivated with ultraviolet irradiation while female mice were immunized by genital tract infection with viable chlamydiae. Arthritis was elicited in all mice by intra-articular inoculation of inactivated MoPn. When knee joints were examined for pathologic changes at varying times after challenge, a marked enhancement of the arthritis was observed in both male and female BCD mice when compared with controls at all time points. These data indicated that the humoral immune response is not essential for the production of arthritic disease in this model but may have some role in the modulation of the process in immunologically intact animals. Persistence of chlamydial antigen in joint tissue of BCD mice suggested that antibody may play a role in the elimination of antigen, thus decreasing the stimulus for the development of cell-mediated immunologic injury. Regulatory role for T suppressor cells cannot be ruled out however, because B cell deficient mice have been shown to lack certain T suppressor cell subsets.