The present study was made to investigate the effect of mizoribine (MZR), an imidazole nucleoside immunosuppressant, on pulmonary lesions and immunological mode of action of MRL/1pr/1pr mice. Four-week-old female MRL/1pr/1pr mice were injected subcutaneously with 20 mg/kg body weight of MZR every other day. For control, mice were given phosphate-buffered saline (PBS) every other day. MZR caused the delay in the histological development of peribronchial and perivascular lymphocytic infiltrations in the lungs of MRL/1pr/1pr (MRL/1pr) mice. And then, MZR suppressed the number of immunoglobulin (Ig) (IgG and IgM) secreting B cells and anti-DNA-secreting B cells in the spleen of MRL/1pr/1pr mice. The above data indicate that MZR could be useful for the treatment of pulmonary lesions associated with autoimmune disorders such as systemic lupus erythematosus (SLE).