Abstract A time-resolved fluorometric assay for the detection of natural killer cell activity against target cells labelled with the fluorescent chelate europium-6,6″-bis[ N, N-bis(carboxymethyl)-aminomethyl]-4'-phenyl-2,2',6',2″-terpyridine (EuCAPT) has been developed. In the assay released EuCAPT from lysed K-562 cells is measured in the supernatant after co-incubation of the target cells with effector cells. Thus, the performance of the assay is essentially similar to the previously described EuDTPA assay and the widely used 51Cr assay. EuCAPT is released from target cells lysed by effector cells faster than 51CrO 4 2− but somewhat slower than EuDTPA. In contrast to methods based on prompt fluorometry the autofluorescence from culture medium supplemented with serum can be avoided by the use of time-resolved fluorometry. The result shows that fluorescent europium chelates provide an alternative to radioactive markers currently used for the assessment of in vitro cellular cytotoxicity.