The possibility that Histoplasma capsulatum may affect the activities of alveolar macrophages through products of either secretion or lysis was examined. Varying amounts of histoplasmin (HP), the filtrate of cultures of H. capsulatum, were added to monolayer cultures of alveolar macrophages of Wistar rats, and the effects of the filtrate on viability, ingestion rate, random migration, adhesive properties and microbial killing capacity were evaluated. No clear cytotoxic or cytopathogenic effects were produced at any dose tested. The endocytic rate was enhanced when HP (0.2 micrograms) was added, the effect being due to the recruitment of non-phagocytosing cells. Alveolar macrophages that had remained nonadherent after 3 h of incubation adhered to glass slides after HP was added. Dose-dependent inhibition of random migration of macrophages was produced with the addition of HP. However, the microbial killing capacity was not modified significantly at any dose of the H. capsulatum product.