Activation of primary human T cells by anti-CD3 and interleukin-2 resulted in partial processing of procaspase-3 in activated nonapoptotic (Delta Psi(m)high) CD8(+) T cells but not in CD4(+) T cells. Apical caspases-8 and -9 were not activated, and Bid was not processed to truncated Bid. Boc-D.fmk, a broad spectrum caspase inhibitor, did not prevent this process, whereas GF.dmk, a selective inhibitor of dipeptidyl peptidase I, was effective. Dipeptidyl peptidase I is required for the activation of granule-associated serine proteases. It is enriched in the cytolytic granules of cytotoxic lymphocytes, where it promotes the proteolytic activation of progranzymes A and B. Inhibition of granzyme B (GrB)-like serine proteases by Z-AAD.cmk prevented partial processing of procapase-3, whereas inhibition of GrA activity by D-FPR.cmk had no effect. Specific inhibitors of other lysosomal proteases such as cathepsins B, L, and D did not interfere in this event. Patients with Chediak-Higashi syndrome or with perforin deficiency also displayed partial processing of procaspase-3, excluding the involvement of granule exocytosis for the delivery of the serine protease in cause. The p20/p12 processing pattern of procaspase-3 in our model points to GrB, the sole serine protease with caspase activity. Small amounts of GrB were indeed exported from cytolytic granules to the cytosol of a significant fraction of GrB-positive cells.