Sensitized BALB/c mice challenged i.p. with 1 microgram of OVA showed IL-5 release in the peritoneal lavage fluid, which peaked at 6 h and decreased thereafter. This was followed by a massive eosinophil accumulation, which started at 6 h and reached a plateau between 24 and 48 h. The i.p. injection of recombinant murine (rm) IL-10 (0.01-0.1 microgram/cavity) along with OVA reduced IL-5 release at 6 h and allergic eosinophilia at 6, 24, and 48 h. rmIL-10 also blocked in vitro IL-5 generation by sensitized peritoneal cells cultured in the presence of OVA. The inhibitory effect of rmIL-10 on Ag-induced eosinophilia and IL-5 release was suppressed by pretreatment of the animals with 1 mg/mouse of a neutralizing anti-mIL-10 mAb. Flow cytometric analysis revealed an increase in the number of CD4+ and CD8+ T lymphocytes and in the number of CD25+/CD4+ cells in the peritoneal lavage fluid collected 24 and 48 h after challenge, respectively; these numbers were reduced significantly by the administration of 0.1 microgram of rmIL-10. Finally, rmIL-10 failed to modify the anti-CD3-induced IL-5 release in vivo in the peritoneal cavity and in vitro from purified spleen CD4+ T lymphocytes. This suggests that rmIL-10 acts indirectly, by deactivating APC, rather than directly on T cell activation. These findings indicate that rmIL-10 displays anti-allergic activity in sensitized BALB/c mice by preventing Ag-induced CD4+ T lymphocyte and eosinophil accumulation as well as IL-5 release in the peritoneal cavity.