Heat shock proteins are major targets of the immune response to bacterial and parasitic pathogens. Mycobacterium tuberculosis hsp70 is an especially powerful Ag containing multiple B and T cell epitopes. We investigated whether M. tuberculosis hsp70 can be used as an adjuvant-free carrier to stimulate the humoral and cellular immune response to an accompanying protein. A recombinant hsp70 protein expression vector was developed that permits the production of any protein fused to the amino terminus of mycobacterial hsp70. We found that a recombinant HIV p24-hsp70 fusion protein produced with this vector elicited both humoral and cellular immune responses against p24 in mice when administered in saline in the absence of adjuvant. Covalent linkage of hsp70 to p24 was essential to elicit immune responses to p24 under these conditions. The anti-p24 IgG1 Abs induced in p24-hsp70-immunized mice persisted at high levels for more than 1 yr after immunization. These results demonstrate that the antigenic properties of M. tuberculosis hsp70 can be exploited to enhance the humoral and cellular immune response to an attached protein.