The interaction of Cryptococcus neoformans with the murine macrophage-like cell line J774.16 was studied in the presence and absence of monoclonal antibodies (MAbs) to the capsular polysaccharide glucuronoxylomannan (GXM). In the absence of MAb 2H1 to GXM, coincubation of J774.16 cells with C. neoformans reduced fungal colony-forming units in only 26.6% of 21 independent experiments. In the presence of MAb 2H1, coincubation of J774.16 cells with C. neoformans reduced fungal colony-forming units in > 95% of experiments. Comparison of the relative efficacy of two IgG1 MAbs revealed that the higher affinity MAb was more effective at low concentrations. Antibody-mediated reductions of C. neoformans colony-forming units by J774.16 cells occurred despite inhibition of nitric oxide synthase, addition of reactive oxygen intermediate scavengers, or the use of a superoxide-deficient J774 mutant line.