The effect of in vivo O3 exposure on the mobility of pulmonary alveolar macrophages (PAM) in vitro was investigated. Eight randomly selected rats were exposed for 4 h. Four rats were exposed to a clean air (sham) atmosphere, and four to an atmosphere containing 1ppm O3. PAM were obtained by lung lavage and placed on gold-colloid coated coverslips. The area cleared of gold particles by migrating PAM after 24, 48, and 72 h was used as an indicator of cell mobility. The number of PAM recoverable by lavage was similar for both groups (2 x 10(5)), but the percentage of macrophages that made tracks was significantly smaller with 95% certainty in the O3 group. For sham-exposed and O3-exposed groups, the area cleared by PAM increased as the length of incubation increased, with the area cleared by the sham-exposed group being about 50% greater during each time period. When the two groups were compared statistically at each time point, the probability that they differed was, in each case, greater than 95%. It was concluded that the in vitro migrational potential of PAM was most likely decreased by in vivo exposure to O3.