Sulfamethoxazole (SMX) is a widely used antibiotic which has been detected in surface water samples in the ng/L range and also detected in drinking water samples. To limit the environmental impact, ozonation treatment of waste streams has been proposed. However, the degradation products created by ozonation as well as their toxicity have not been reported. In this study, we investigated the degradation products of SMX formed during ozonation and the effects of these products on mammalian cultured cells. In addition to alcohols and nitrates, sulfanilamide was identified as the larger molecular weight compound of the degradation products detected. Cells exposed to the degradation products of SMX maintained their polyhedral geometry longer than the control cells. Proliferation of the cells exposed to the degradation products was not negatively affected when compared with the control cells. The results of this study show that bioactive degradation products can be formed by ozonation of SMX.