Clones were obtained from highly purified populations of human peripheral blood natural killer (NK) cells propagated in the presence of interleukin-2 and phytohaemagglutinin. Almost all clones were cytotoxic against standard NK targets and many were also able to kill the B lymphoblastoid cell line BSM. This latter property was not necessarily a result of the incorporation of this cell line into the feeder mixture used to derive the clones. In most cloning experiments there was a high degree of concordance between the killing of the NK targets K562 and Molt 4 by panels of clones. In some cases this extended to the killing of BSM targets but in other instances there was no relationship or even an inverse correlation between killing of BSM and other targets. In a single cloning experiment there was no relationship between killing of BSM and Raji targets. In some cases a panel of clones could be divided into two or more distinct groups based on their differential activity towards BSM and K562. Such differences were not solely due to inter-donor variation. These findings were extended by cold target inhibition experiments in which at least three types of clone were identified. In one group of clones, which was nonreactive towards BSM, cold BSM significantly enhanced the killing of K562 in a dose-dependent fashion. These experiments provide evidence for a limited degree of functional heterogeneity among clones derived from human peripheral blood NK cells.