Supernatants from bovine immunodeficiency-like virus (BIV)-infected cells have been previously shown to affect monocyte random migration, chemotaxis, phagocytosis, and antibody-dependent cell-mediated cytotoxicity (ADCC) in vitro. The experiments in this report demonstrate that the BIV Gag (core) proteins can enhance monocyte random migration, chemotaxis, and phagocytosis. Supernatants from BIV-infected cells contained 10-30 and 30-50 kDa proteins, which significantly (p < 0.05) increased monocyte chemotaxis. The 30-50 kDa protein(s) could be cleaved by limited proteolysis into 10-30 kDa active components. Affinity purification with monoclonal anti-p26 (capsid) antibodies yielded preparations that were active in the random migration, chemotaxis, and phagocytosis assays, but did not affect ADCC. Furthermore, activity of the affinity purified preparation could be specifically neutralized by hyperimmune rabbit serum against BIV Gag proteins. A recombinant Gag protein, consisting primarily of BIV p26, also enhanced monocyte random and chemotactic migration. It appears, therefore, that direct treatment with affinity-purified BIV Gag proteins or a recombinant Gag protein, is able to significantly affect the function of normal monocytes in vitro. Factors affecting monocyte migration and phagocytosis appear to be one or more breakdown products of the BIV Gag precursor, particularly those containing the p26 (capsid) protein.