Isobutyl nitrite is a drug of abuse popular among male homosexuals and among adolescents. In order to approximate the nitrite exposures of inhalant abusers, mice were treated with 900 ppm isobutyl nitrite in an inhalation chamber for 45 min per day for 14 days. After 14 consecutive days of exposure to isobutyl nitrite, mice weighed an average of 4% less than mice exposed to air. The spleens of nitrite-exposed mice weighed 15% less and had 24% fewer cells per spleen than those of controls. Adjusted for equal cell numbers, T cell mitogenic and allogeneic proliferative responses were significantly reduced by 33 and 47%, respectively. The frequency of T-dependent plaque-forming cells (PFC) was inhibited by 63% and the total number of PFC per spleen was reduced by 72% in nitrite-exposed mice. In contrast, B cell proliferative responses to LPS were unaltered, suggesting that the toxicity of isobutyl nitrite did not affect all lymphoid cells equally. The data suggest that habitual inhalation of isobutyl nitrite could impair immune competence and that toxicity appeared to be directed toward T cell functions.