A fluorescence assay for the quantitation of tumor cell lysis by activated and non-activated killer (LAK) cells is described. The target cells are labelled with a europium chelate (Eu-diethylenetriaminopentaacetate) and after cytolysis caused by the LAK cells the Eu3+ complex is released into the culture supernatant. The addition of beta-naphthoyltrifluoroacetone to culture supernatant aliquots leads to the formation of a highly fluorescent chelate which can be measured with a time-resolved fluorometer. The influence of various assay parameters has been evaluated including incubation time, effector-to-target cell ratio, the target cell line and different concentrations of interleukin-2 during cell culture. The optimized time-resolved fluorometric assay was found to be as simple and sensitive as the commonly used cytotoxicity assay in which the release of 51Cr from the labelled target cells is measured. In addition the assay is much faster and safer since the label is not radioactive.