A sister chromatid exchange (SCE) analysis was performed on circulating lymphocytes collected from young human volunteers exposed to 0.5 ppm ozone for 2 h, an exposure simulating conditions in severe air pollution episodes. SCE analysis was also performed on diploid human fetal lung cells (WI-38) exposed to 0, 0.25, 0.50, 0.75 and 1.00 ppm ozone for 1 h in vitro. No increase in SCE over controls was demonstrated in the lymphocyte chromosomes. A clear dose-related increase in SCE was observed in the WI-38 cells exposed in vitro.