The protozoan parasite Toxoplasma gondii is increasingly recognized as a waterborne pathogen. Infection can be acquired by drinking contaminated water and conventional water treatments may not effectively inactivate tough, environmentally resistant oocysts. The present study was performed to assess the efficacy of 2 commonly used chemicals, sodium hypochlorite and ozone, to inactivate T. gondii oocysts in water. Oocysts were exposed to 100 mg/L of chlorine for 30 min, or for 2, 4, 8, 16, and 24 hr, or to 6 mg/L of ozone for 1, 2, 4, 8, or 12 min. Oocyst viability was determined by mouse bioassay. Serology, immunohistochemistry, and in vitro parasite isolation were used to evaluate mice for infection. Initially, mouse bioassay experiments were conducted to compare the analytical sensitivity of these 3 detection methods prior to completing the chemical inactivation experiments. Toxoplasma gondii infection was confirmed by at least 1 of the 3 detection methods in mice inoculated with all doses (10(5)-10(0)) of oocysts. Results of the chemical exposure experiments indicate that neither sodium hypochlorite nor ozone effectively inactivate T. gondii oocysts, even when used at high concentrations.