Extensive efforts have recently been made to characterize cells capable of mediating natural killing activity (see ref. 1 for review) and increasing evidence has arisen that these cells were heterogeneous. By using the methods we have recently developed for cloning natural killer (NK) cells derived from peripheral blood, we have analysed the heterogeneity of human NK cells. Seven cell lines showing NK activity were established and studied for 4 months. Their phenotype was determined with a series of monoclonal antibodies; anti-T1, -T3, -T4, -T8, -T11, -T12, Mo1 and each cell line appeared to have a unique phenotype. Moreover, whereas some of these lines could only kill K562 cells, the standard assay of NK activity, others displayed a broad but distinct spectrum of reactivity against a variety of human tumour and viral transformed cell lines. Taken together, these results demonstrate the phenotypic and functional heterogeneity of NK effector cells which has recently been suggested in both human and murine systems.