NZB/NZW F¹ hybrid mice develop a spontaneous autoimmune disease characterized by the appearance of antinuclear antibodies and premature death due to immune complex glomerulonephritis. To investigate the possible effects of cellular immune stimulation on this disorder, groups of female NZB/NZW mice, aged 2, 5, and 7 months, were treated either with the nonspecific immunostimulatory agent Mycobacterium bovis strain BCG or with saline. Mice treated with BCG at ages 5 and 7 months died sooner than age‐matched controls, and death was associated with severe glomerulonephritis, suggesting that BCG may have accelerated autoimmunity in these mice. Since BCG is known to stimulate the production of type II (or γ) interferon, a substance with potent immunoregulatory effects, a second study was carried out to assess the effects of type II interferon on NZB/NZW disease. A greater number of type II interferon‐treated mice died by 9 months of age when compared to controls, and the increased death rate was associated with a more rapid development of antinuclear antibodies and histologically confirmed glomerulonephritis. These data, together with a recent report of increases in the level of serum type II interferon in patients with active systemic lupus erythematosus, suggest that type II interferon may play a role in the pathogenesis of autoimmune disease.