In the first phase of this study the natural cytotoxic activity of human extravascular lymphoid tissue was examined. In contrast to some previous investigations it is reported that the human palatine tonsil contains active natural killer (NK) cells. Like peripheral NK cells, tonsil cytotoxic lymphocytes possess a low buoyant density which allows their enrichment from non-cytotoxic cells. However, in contrast to blood borne natural killier cells which exhibit a large granular morphology, tonsil NK cells are agranular. Furthermore a functional distinction from classical NK cells is apparent, since although the cytotoxic activity of tonsil natural killer cells can be augmented with various immunopotentiating agents including interleukin 2, exposure to interferon (IFN)-a fails to influence reactivity. These data provide evidence for heterogeneity among human NK cells. The existence of NK cell resident within or recirculating through extravascular lymphoid tissue was confirmed by studies of axillary-and mesenteric lymph nodes. Cell populations isolated from these tissues were found to exhibit levels of NK activity equivalent to, or greater than, that observed with tonsil lymphocytes. In contrast to tonsil, NK cells, however, the cytolytic potential of some lymph node cell preparations could be significantly enhanced by IFN-a. It is suggested that natural killer cells within extravascular lymphoid tissues play an important role in providing systemic innate immunity.RThe second objective of this study was to examine the mechanisms of target cell resistance to natural cytotoxicity. Cloned variants of the human erythroleukaemic cell line K562 were isolated by limiting dilution and found to exhibit marked and stable differences in their resistance to NK lysis. Detailed examination of two such clones [E10/P2 and F9/P2] revealed that the observed variation in susceptibility to natural cytotoxicity was not attributable to differential expression of NK recognition determinants. In contrast to other studies in which isolated or induced target cell variants have been shown to exhibit a selective resistance to lysis by NK cells, the resistant clone [F9/P2] examined in this investigation was also less susceptible to antibody-dependent cellular cytotoxicity and complement-mediated lysis. These data indicate that mechanisms exist whereby a cell may exhibit reduced susceptibility to natural cytotoxicity through a generalised capacity to resist immunolytic processes. Cloned variants of K562 were also employed to examine the effect of differentiation-inducing agents, 12-0-tetradecanoylphorbol-13-acetate [TPA) and sodium n-butyrate [NaB] on resistance to NK lysis. It is reported that both TPA and NaB caused increased sensitivity of the resistant variant F9/P2. In contrast TPA, but not NaB, increased the resistance of the sensitive clone, E10/P2 to natural cytotoxicity. The data reveal that differentiation-induced alterations in sensitivity to NK lysis are variable among clones of the same cell line.