A few cases have been described of antigenic determinants that are broadly presented by multiple class II MHC molecules, especially murine I-E or human DR, in which polymorphism is limited to the beta chain, and the alpha chain is conserved. However, no similar cases have been studied for presentation by class I MHC molecules. Because both domains of the MHC peptide binding site are polymorphic in class I molecules, exploring permissiveness in class I presentation would be of interest, and also such broadly presented antigenic determinants would clearly be useful for vaccine development. We had defined an immunodominant determinant, P18, of the HIV-1 gp160 envelope protein recognized by human and murine CTL. To determine the range of class I MHC molecules that could present this peptide and to determine whether two HIV-1 gp160 Th cell determinants, T1 and HP53, could also be presented by class I MHC molecules, we attempted to generate CTL specific for these three peptides in 10 strains of B10 congenic mice, representing 10 MHC types, and BALB/c mice. P18 was presented by at least four different class I MHC molecules from independent haplotypes (H-2d, p, u, and q to CD8+ CTL. In H-2d and H-2q the presentation was mapped to the D-end class I molecule, and for Dd, a requirement for both the alpha 1 and alpha 2 domains of Dd, not Ld, was found. HP53 was also presented by the same four different class I MHC molecules to CD8+ CTL although at higher concentrations. T1 was presented by class I molecules in three different strains of distinct MHC types (B10.M, H-2f; B10.A, H-2a; and B10, H-2b) to CTL. The CTL specific for P18 and HP53 were shown to be CD8+ and CD4- and to kill targets expressing endogenously synthesized whole gp160 as well as targets pulsed with the corresponding peptide. To compare the site within each peptide presented by the different class I molecules, we used overlapping and substituted peptides and found that the critical regions of each peptide are the similar for all four MHC molecules. Thus, antigenic sites are broadly or permissively presented by class I MHC molecules even without a nonpolymorphic domain as found in DR and I-E, and these sequences may be of broad usefulness in a synthetic vaccine.