Two immunoglobulin M monoclonal antibodies (mAbs) derived from the same B cell recognize different epitopes on the capsular polysaccharide of the pathogenic yeast, Cryptococcus neoformans. Their respective epitopes are located in spatially distinct regions of the capsule. Passive administration of one mAb prolonged survival whereas the other mAb did not. The results indicate that specificity is an important determinant of antibody efficacy against C. neoformans and that somatic mutations occurring during the antibody response can affect the protective efficacy of antibodies to C. neoformans.