We have examined the regulation of complement dependent phagocytosis by macrophage-activating cytokines. Tumor necrosis factor (TNF)-alpha and granulocyte-macrophage colony-stimulating factor (GM-CSF), but not interferon-gamma, interleukin-4 or macrophage-CSF, stimulated ingestion of the encapsulated fungal pathogen Cryptococcus neoformans by resident peritoneal macrophages in vitro. This was dependent upon opsonization of the yeasts with complement, 72 h of incubation with the cytokines for maximum effect, and the obligate involvement of the macrophage CR3 receptor. TNF-alpha and GM-CSF synergized at low concentrations, resulting in dramatic up-regulation of phagocytosis when compared to either cytokine alone. Supernatants from C. neoformans-specific T cells also increased macrophage phagocytic efficiency. Finally, the administration of neutralizing mAb specific for TNF-alpha and GM-CSF increased mortality in C. neoformans-infected mice, and induced the rapid progression of disease with involvement of the brain and meninges. We conclude that TNF-alpha and GM-CSF are potent regulators of complement-dependent phagocytosis by murine macrophages. Macrophage activation with these two cytokines can completely overcome the anti-phagocytic properties of the virulent yeasts. Our results, therefore, implicate TNF-alpha and GM-CSF as important mediators of resistance to encapsulated pathogens such as C. neoformans where ingestion of the organism is a critical process in host resistance.