Synthetic peptides are widely used to define the specificity of CD8+ cytotoxic T-lymphocyte (CTL) clones. When many peptides need to be tested by the standard chromium release assay large numbers of a CTL clone are required. Specific synthetic peptide epitopes induce CTL clones to kill each other. This phenomenon can be directly visualized using an inverted microscope and forms the basis for a convenient assay, which can be performed with as few as 100 CTL per peptide and does not require radiolabelled targets.