In this study we investigated the involvement of inflammatory cells in the pleural accumulation of eosinophils induced by lipopolysaccharide (LPS). Intrathoracic (i.t.) injection of LPS (250 ng/cavity) into rats induced a significant eosinophil accumulation that developed within 24 h, was maximal at 48 h, and returned to control values within 120 h. This eosinophil influx was preceded by a huge neutrophil influx within 4 h and accompanied by a mononuclear cell accumulation between 24 and 48 h. Pretreatment with an antineutrophil monoclonal antibody (RP-3, 2 ml per animal) selectively reduced the number of circulating neutrophils within 8 h but failed to alter the LPS-induced eosinophilia. Similarly, platelet depletion with an anti-rat platelet antiserum did not alter the LPS-induced eosinophil accumulation. Cyclosporine (50 mg/kg, 12 and 2 h before) partially inhibited (51%) the LPS-induced pleural eosinophilia, whereas the eosinophilia was not changed by prior degranulation of pleural mast cells with polymyxin B (10 micrograms/cavity, 24 h before). Moreover, selective depletion of T lymphocytes using an anti-Thy 1.0 monoclonal antibody significantly inhibited the eosinophilia triggered by LPS. The i.t. injection of liposomes containing dichloromethylene diphosphonate significantly reduced (65%) the number of resident macrophages after 5 days. Under this condition, the eosinophil infiltration induced by LPS was completely inhibited. Accordingly, the i.t. injection of supernatant from macrophage monolayers, obtained from the pleural cavities of LPS-injected rats, into naive recipient animals led to a twofold increase in the number of pleural eosinophils. In conclusion, our data suggest an important role for resident macrophages and T lymphocytes in the eosinophil accumulation induced by LPS.