We have examined the immunogenicity of complexes formed by non-covalent association of a synthetic peptide corresponding to influenza A virus nucleoprotein, residues 206-229 (pNP) and Mycobacterium tuberculosis heat-shock protein 70 (hsp 70). One or two injections of these complexes given to BALB/c mice without any additional adjuvant, were capable of eliciting very strong peptide-specific proliferative T-cell responses in the spleen. These responses were dependent on the stability of the complex since immunogenicity was lost when dissociated with ATP prior to immunization. T-cell responses to hsp 70 were easily generated by immunization with the purified chaperone alone, either after primary or secondary immunization. Injection of pNP-hsp 70 complexes, however, although generating good primary responses, resulted in very much decreased proliferative responses to the hsp 70.