An extract of the cell wall and cell membrane from Histoplasma capsulatum yeast cells was assayed by Western blot (immunoblot) for reactivity with two monoclonal antibodies to heat shock protein 70. Four bands with molecular masses of 80, 66, 54, and 32 kDa bound both antibodies. The 80-kDa protein was isolated, analyzed for homology to heat shock protein 70, and tested for antigenicity and immunogenicity in C57BL/6 mice. The 80-kDa protein reacted with monoclonal antibody to heat shock protein 70. Sera from mice immunized with the antigen recognized H. capsulatum heat shock protein 70. Moreover, the amino-terminal sequence of the 80-kDa protein revealed substantial homology with heat shock protein 70 from several species. The 80-kDa protein induced delayed-type hypersensitivity responses in mice immunized with either viable yeast cells or antigen. Splenocytes from mice immunized with yeast cells or with antigen responded in vitro to the 80-kDa antigen. Immunization of mice with the antigen enhanced host resistance against a sublethal inoculum of H. capsulatum yeast cells, but it did not reduce the mortality of mice given a lethal challenge of yeast cells. Thus, this antigen manifests homology with members of the heat shock protein 70 family. Furthermore, the 80-kDa protein elicits cellular immune responses to H. capsulatum, and it mediates protective immunity.