Antigen recognition by T cells is degenerate both at the MHC and the TCR level. In this study, we analyzed the cross-reactivity of a human HIV-1 gag p24-specific CD4(+) T cell clone obtained from an HIV-1-seronegative donor using a positional scanning synthetic combinatorial peptide library (PS-SCL)-based biometrical analysis. A number of decapeptides able to activate the HIV-1 gag-specific clone were identified and shown to correspond to sequences found in other human pathogens. Two of these peptides activated the T cell clone with the same stimulatory potency as the original HIV-1 gag p24 peptide. These findings show that an HIV-1-specific human T helper clone can react efficiently with peptides from other pathogens and suggest that cellular immune responses identified as being specific for one human pathogen (HIV-1) could arise from exposure to other pathogens.