Some targets of cell-mediated cytolysis do not efficiently release 51Cr but manifestly are killed in direct viability assays. We characterize and validate an alternative and non-radioactive (colorimetric) method for measuring killing of adherent targets by monoclonal CTL. The method obviates concerns about the effects of trypssinization, is technically simple, quantitative and in some cases more sensitive than conventional 51Cr assays. Target loss obeyed first-order kinetics with respect both to [CTL] and time. These results are consistent with an exponential (Poisson) model of killing and support the use of a single kinetic parameter to describe the lytic activity of monoclonal CTL on adherent targets. When monoclonal CTL are used at appropriate effector: target ratios (≤ 1:1), the residuals obtained after least squares linear regression are homoscedastic and normally distributed, justifying the use of commonly available statistical calculators or programs for the analysis of CTL data. © 1992.