1 Exposure to elevated levels of ozone results in an infiltration of polymorphonuclear leukocytes (PMNs) into the lungs. The purpose of this study was to investigate whether the ozone-induced inflammatory process is preceded by a change in the expression of adhesion molecules (integrins and selectins) in peripheral blood PMNs and alveolar macrophages in rats. 2 Female Sprague Dawley rats were exposed to air or ozone (1 p.p.m., 2 h). Bronchoalveolar lavage (BAL) was carried out and blood was collected via intracardiac puncture at 0 or 18 h after the exposure. There were no PMN in the BAL fluid at time 0 after the 2 h exposure to ozone. The expression of cell adhesion molecules from the integrin family (represented by CD18) on alveolar macrophages (AM) was lowered. The expression of cell adhesion molecules from the selectin family (represented by CD62L) on blood PMN was not affected by exposure to ozone, while the expression of integrins (CD11b) on blood PMN was lowered. 3 This effect was confirmed by experiments in which plasma of ozone-exposed animals was incubated with PMN from peripheral blood obtained from nonexposed animals. In these experiments, the expression of CD11b on PMNs of non-exposed animals was lower after incubation with plasma from ozone-exposed animals. 4 Our experiments suggest the presence of factor(s) in blood, which cause a decrease in the expression of CD11b on PMNs.