In order to identify T-cell antigens from Histoplasma capsulatum yeast cells, we prepared a detergent extract of the cell wall and cell membrane of yeast-phase H. capsulatum G217B and analyzed its antigenicity and immunogenicity. Mice injected with viable H. capsulatum yeast cells or with 500 or 1,000 micrograms of the extract mounted a delayed-type hypersensitivity response to solubilized cell wall and cell membrane. Vaccination with this antigenic preparation conferred a protective immune response in mice that were challenged intravenously with H. capsulatum yeast cells. The extract induced in vitro proliferation by splenocytes from mice injected with either viable yeast cells or the soluble cell wall and cell membrane preparation. We also examined the profile of in vitro responses by a murine T-cell line and by cloned T cells to soluble cell wall and cell membrane by employing the technique of T-cell immunoblotting. Two prominent regions that stimulated the T-cell line and cloned T cells were identified. Fractions encompassing an area between 53 and 64 kDa caused proliferation by a T-cell line and five of six clones. Antigens recognized by the T-cell line and by three of six clones were contained in another area that extended from 69 to 82 kDa. The data demonstrate that this soluble extract from cell wall and cell membrane contains antigens recognized by T cells and mediates protective immunity. Moreover, T-cell immunoblotting provides a useful technique for mapping immunoreactive molecules from H. capsulatum yeast cells.