The group-specific antigens Pr55gag of human immunodeficiency virus type-1 (HIV-1) self-assemble into noninfectious virus-like particles (VLP) that are released from various eucaryotic cells by budding. Deletion analysis of Pr55gag mutants revealed three domains into which sequences of the third variable domain V3 or the CD4-binding domain of the gp120 external glycoprotein can be inserted without destroying the capacity of the chimeric proteins to assemble to VLP. Immunization of rabbits with different types of purified chimeric VLP without adjuvants raised a strong antibody response to the Pr55gag carrier component. The magnitude of the antibody response to the inserted gp 120 epitopes strictly depended on their position within the gag polyprotein. These antisera exhibited only weak neutralizing activity. However, BALB/c mice immunized by different routes with different types of chimeric Pr55gag/V3 VLP without adjuvants developed a strong MHC class I (Dd)-restricted, cytolytic CD8+ T-cell (CTL) reactivity against a known epitope within the V3 domain. When the recombinant antigen was emulsified in mineral oil (incomplete Freund's adjuvant) or adsorbed in aluminium hydroxide, its immunogenicity for CTL was drastically reduced or completely abrogated. The magnitude of the V3-specific CTL response was not influenced by the position of the V3 domain within the Pr55gag-carrier moiety; the flanking residues, hence, did not influence processing of the exogenous antigen for MHC class I-restricted peptide presentation. These results indicate ways for the rational design and optimal delivery of CTL-stimulating HIV candidate vaccines.