Melanins are associated with virulence in several important human pathogens, but little is known about the immune response to this ubiquitous biologic compound. We hypothesized that melanin produced by the fungus Cryptococcus neoformans was immunogenic. C. neoformans melanin was purified from melanized fungal cells and was used to immunize C57BL/6, BALB/c, and T cell-deficient (nude) BALB/c mice. The Ab response was evaluated by ELISA, immunofluorescence, and agglutination. The results demonstrate that melanin can be immunogenic, and the humoral immune response is T cell independent. Furthermore, the experiments demonstrate 1) a sensitive ELISA for the measurement of Ab to melanin, 2) that mice mount an intense Ab response to fungal melanin that includes Abs of IgM and IgG isotypes, 3) that melanins from different sources have cross-reactive epitopes, and 4) melanin in the cell wall of melanized yeast cells reacts with Abs raised to L-dopa C. neoformans melanin. The biologic significance of Ab to melanin remains to be determined, but the development of Ab suggests that this amorphous insoluble polymer can stimulate the immune system. The serologic techniques described here may prove useful for the evaluation of Ab responses to melanin in a variety of diseases.