Cytotoxic T cells were induced in vivo by immunizing C57BL/6 mice with a single dose of a short synthetic peptide representing amino acid residues 497-507 from HSV-1 glycoprotein B. Primed lymph node cells did not require in vitro boosting with peptide and APC for CTL detection. The CTL were CD8+ and H-2b restricted. They were capable of lysing target cells exogenously sensitized with peptide or endogenously processed glycoprotein B. Virus-primed CTL produced an anamnestic response in vivo upon peptide challenge, indicating that peptide-specific CTL may be relevant to infection. The requirement of CD4+ T cells for CD8+ CTL activation was investigated by depleting CD4+ cells in vivo with GK1.5 mAb. CD4+ T cell depletion did not abrogate CTL generation. These results suggest that glycoprotein B peptide 497-507 activates CD8+ CTL in vivo in a manner independent of CD4+ T cells. This is the first study in which a class I-restricted peptide derived from a HSV protein was used to activate CTL in vivo, and in which a synthetic peptide was shown to activate CTL independent of CD4+ T cell help. Our data are relevant to viral vaccine development and to processing and presentation of viral epitopes.