Exposure of mice to 1.96 mg/cu. m. ozone (03) 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 compared to controls. Exposure of guinea pigs to 2.9 mg/cu. m 03 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/cu. m 03 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.