The use of a new marker for labelling cells used as targets for natural killer cells is described. The human erythroleukaemic cell line K-562 was used as target. The cells were labelled with europium diethylenetriaminopentaacetate (EuDTPA) chelates. The detection of the released marker is based on time-resolved fluorometry. The results obtained show that the method is sensitive, specific and rapid. The high specific activity of the marker and the sensitivity of the detection apparatus result in numeric values (counts per second) which are 10-20 times higher than the values (counts per minute) obtained with 51chromium. In comparison with 51chromium release assay the labelling of target cells is less time consuming, the marker release more rapid and the detection time of released marker only 1 second per tube. The use of this non-radioactive marker is an alternative way of measuring natural killer cell mediated cytolysis of target cells.