Immune responses, including lymphocyte transformation and protection, were evaluated in a new model of experimental murine cryptococcosis. Newly developed, genetically stable, acapsular mutants of Cryptococcus neoformans were used to immunize C3H/HeN mice. Intraperitoneal and subcutaneous routes of immunization without adjuvant were studied. Spleen cells and pooled popliteal and inguinal lymph node cells responded significantly to heat-killed, whole-cell C. neoformans antigen but not to Candida albicans antigen in lymphocyte transformation assays. No significant difference in lymphocyte transformation response to antigens was observed between intraperitoneally and subcutaneously immunized animals. Mean survival time of immunized mice was extended significantly in comparison to controls following intravenous challenge with virulent C. neoformans. Degree of protection was independent of route of immunization but dependent upon the magnitude of the challenge inoculum. This model should prove valuable in the study of immunoregulation of cryptococcosis and vaccine development.