A major goal in immunodiagnostics has been the development of assay systems that can measure CD8(+) T cell immunity in humans, directly ex vivo, at high resolution, and with high throughput. We established granzyme B (grB) enzyme-linked immunospot assay (ELISPOT) in conjunction with image analysis to this end. Using grB transfected and untransfected Chinese hamster ovary (CHO) cells and T cell lines, we show that the antibody pair utilized was grB-specific and that only activated T cells secrete grB. GrB release began within 4 h after antigen stimulation and stopped within 40 h. Side-by-side comparison showed grB ELISPOT assays to have a higher resolution than classic chromium-release assays in terms of signal-to-noise ratio. The linearity of the relation of the number of CD8(+) effector T cells plated to grB spots detected suggests that grB ELISPOT assays measure the frequencies of grB-secreting cells directly. Reactivity to HIV peptides was seen in grB ELISPOT assays of freshly isolated PBMC from HIV patients, consistent with the detection of peptide-specific memory cells. The higher resolution and lower labor and material investment should make grB ELISPOT assays an attractive alternative to chromium-release assays in monitoring the clonal sizes of specific CD8 memory cells in vivo.