This research program was initiated with the overall objective of determining whether exposure to ozone could damage the DNA of peripheral blood cells. An animal model system was designed in which glycogen was used to stimulate the production of peritoneal exudate cells (PECs) in mice. These PECs were labeled by repeated i. v. injection of 3H-thymidine. The labeled PECs circulated briefly through the peripheral blood and eventually accumulated in the peritoneal cavity. The experimental animals were either exposed to ozone or ambient atmosphere (sham treated) during the period of time when the PECs were circulating through the peripheral blood. These PECs were then harvested and their DNA analyzed for single strand breaks on alkaline sucrose gradients. At very high levels of ozone exposure (5 ppm 9800 micrograms/cu m 24 hours) DNA damage was readily apparent. At lower levels of ozone (1 ppm for 24 hours), some DNA damage may have occurred. In each experiment at this level, some decrease in the average molecular weight of the DNA from ozone exposed animals was detected. However, the data were not statistically significant in terms of increased number of single strand breaks. No attempt was made to assess DNA damage at even lower ozone levels.