Granules released by cytotoxic T cells (CTL), during recognition and killing of target cells, contain granule enzyme A. This serine protease has an esterase activity, which is easily measured using the substrate benzyloxycarbonyl-L-lysine thiobenzyl ester (BLT). BLT activity, routinely used as an assay for granule release, provides an alternative to the standard chromium release assay as a measure of CTL-mediated killing. The two methods were highly comparable when either exogenous synthetic peptide or endogenously produced epitopes were used as targets and human CTL clones acted as effectors. The advantages of the BLT assay are that it uses inexpensive non-radioactive reagents, the assay can be run over any period between 4 and 30 h and can be performed with as few as 10(4) CTLs if synthetic peptide epitopes are used.