The form in which antigens are transferred from cancer cells or infected cells to antigen-presenting cells as a part of the process of priming CD8(+) T cells has been a longstanding unresolved issue. Intact proteins or protein fragments in the form of free peptides or peptides chaperoned by heat-shock protein are possible sources of antigen. We address this here using beta-galactosidase and ovalbumin. Immunization with cell lysates containing intact proteins and heat-shock protein-peptide complexes or with cell lysates depleted of either component demonstrated that protein fragments chaperoned by heat-shock protein and not intact protein were the necessary and sufficient source of antigen transferred to antigen-presenting cells for priming CD8(+) T cell responses.