Exposure of mice to 1.96 mg/m3 ozone (O3) 3 h/day, 5 days/week, for up to 8 weeks beginning at 1 or 2 weeks after challenge with Mycobacterium tuberculosis R1Rv resulted in significant enhancement of bacterial titers in the lungs at 5 through 8 weeks after challenge when compared to mice exposed to filtered air. Exposure to lower concentrations of O3 did not produce any significant changes compared to controls. Exposure of guinea pigs to 2.9 mg/m3O3 for 3 h immediately after challenge with M. tuberculosis resulted in a suppression of the cutaneous delayed hypersensitivity response, without affecting the serum hemagglutination antibody titers. However, exposure of guinea pigs to 0.98 mg/m3 O3 3h/day for 5 days, initiated within 3 h after the infectious challenge, enhanced hemagglutination antibody titers initially, but the delayed hypersensitivity reaction did not differ from controls.