Heat shock protein 70 (hsp70) is a potent adjuvant that links innate and adaptive immune responses. To study how hsp70 activates naive CD8+ T cells in vivo, we tracked Ag-specific CD8+ T cells in mice immunized with a fusion protein containing chicken OVA linked to hsp70 derived from Mycobacterium tuberculosis (OVA.TBhsp70). On a molar basis, OVA.TBhsp70 was several hundred times more effective than OVA peptide plus CFA in eliciting specific CD8+ T cell responses. Immunization with OVA.TBhsp70 activated >90% of detectable OVA-specific CD8+ T cells within 3 days and led to the persistence of cytotoxic effectors for at least 17 days. These studies demonstrate that the potent adjuvant effect of M. tuberculosis hsp70 results from the relatively complete, rapid, and durable activation of Ag-specific CD8+ T cells.