The NK and K-cell activity of human leukocytes was investigated as compared with those cells of the K 562 cell line and murine cells covered by xenoantibodies in Graffi erythroblast leukaemia by means of the 51Cr release test. NK and K-cells could be identified in the blood and bone-marrow. However, they could not be identified in the thymus, lymph-nodes, and tonsils. Attempts of cell fraction with the blood of healthy donors revealed that the K-cells must be attributed to non-T-lymphocytes. NK-cells may be found in the fraction of non-T-lymphocytes as well as in that of T-lymphocytes. Killer cell activity tests in children with acute leukaemia resulted in leukaemia cells having NK and K-cell activity only in very rare cases. ALL patients in remission had strongly lowered NK-cell values under chemotherapy. In comparison to that, chemotherapy had no influence on K-cell activity. On the one hand, NK-cell activities were induced in mixed cultures of allogenous lymphocytes of the blood and, on the other hand, in cells of lymph-nodes. Attempts of fractionation, investigations for determining the influence of chemotherapy and attempts of inducing killer cell activity in vitro lead to the conclusion that NK and K-cells may be regarded as similar cell populations, being, however, not identical.