Macrophage cytotoxicity for Cryptococcus neoformans was investigated by culturing human alveolar macrophage (AM) with a thln-capsuled clone of C. neoformans In a polypropylene culture tube assay system. Yeast replication was quantitated by electronic particle counting after detergent lysis of AM and viability by quantitative plate counts. Under appropriate conditions, fungal replication was Inhibited in the presence of human AM. This effect persisted over the 48-h time course that wasevaluated. During this period, organisms In medium alone proliferated rapidly, doubling their number every 4 h. Human AM did not require endotoxin, fetal calf serum, or specific rabbit antlcryptococcal antibody for funglstasis. Under these conditions, microscopic evaluation of a cytocentrifuge preparation of AM-yeast cocultures, stained by a modified Giemsa technique, revealed all the fungi to be extracellular. In the presence of 10% fresh human serum, AM phagocytized C. neoformans and exhibited fungicidal activity. Tumor necrosis factor did not affect the replication rate of the yeast. These findings suggest that there may be at least 2 mechanisms by which human AM protect against C. neoformans. One is serum-independent and extracellular and results in funglstasls, and the other is dependent on a serum factor and leads to intracellular inhibition of growth and possibly killing of the organism. AM REV RESPIR DIS 1987; 136: 1242-1247