The cellular sources of leukoregulin (LR) and lymphotoxin (LT) from humanperipheral blood leukocytes or spleen cells were examined.The ratios of LT activity to LRactivity in lymphokine preparations from different individuals were not constant but variedover 640-fold,suggesting that LR and LT activities were mediated by distinctly differententities.Purified LR was used to examine the relative susceptibilities of human or micecells to LR.Most human tumor cells were very sensitive to LR while human normal cellsand mice normal or tumor cells were less sensitive or resistant.Crude preparations ofhuman interferon Hu IFN-γ or Hu IFN-α were more cytotoxic to human tumor cells thanpartially purified natural Hu IFN-γ or highly purified rHu IFN.αD,and crude or purifiedLR were more cytotoxic to human tumor cells than Hu IFN-γ or Hu IFN-α.Hu IFN-γactivity but not cytotoxic activity could be removed from preparations containing Hu IFN-γ,LR and LT activities with the aid of monoclonal antibody to Hu IFN-γ.Natural Killercytotoxic factor (NKCF) supernatants from the co-culture of human spleen cells and NKsusceptible cell line K562 or Molt-4 were strongly cytotoxic to various tumor cell lines.TheNKCF activity was completely inactivated by incubating at pH 2 for 24 h.The putativereceptor for LR or LT was studied using adsorption assay.The present study reveals thathuman LR activity can be adsorbed from crude LR supernatants by incubation with nativeor formalin-treated K562 or SMMC.7721 cells at 37℃ or 4℃;LT activity can be adsorbedwith L929 cells.Therefore,LR appears to be a cytotoxic/cytostatic factor that is distinctfrom LT,IFN and NKCF. The cellular sources of leukoregulin (LR) and lymphotoxin (LT) from humanperipheral blood leukocytes or spleen cells were. The ratios of LT activity to LRactivity in lymphokine preparations from not continuously and variedover 640-fold, suggesting that LR and LT activities were mediated by distinctly differententities. Purified LR was used to examine the relative susceptibilities of human or mice cells to LR. Host human tumor cells were very sensitive to LR while human normal cells and mice normal or tumor cells were less sensitive or resistant.Crude preparations of human interferon Hu IFN-γ or Hu IFN-α were more cytotoxic to human tumor cells than partially purified natural Hu IFN-γ or highly purified rHu IFN.αD, and crude or purifiedLR were more cytotoxic to human tumor cells than Hu IFN-γ or Hu IFN-α. Hu IFN-γactivity but not cytotoxic activity could be removed from preparations containing Hu IFN-γ, LR and LT activities with the aid of monoclonal antibod y to Hu IFN-γ.Natural Killercytotoxic factor (NKCF) supernatants from the co-culture of human spleen cells and NKsusceptible cell line K562 or Molt-4 were strongly cytotoxic to various tumor cell lines. The NKF activity was completely inactivated by incubating at pH 2 for 24 h. The putative receptor for LR or LT was studied using adsorption assay. The present study reveals that human LR activity can be adsorbed from crude LR supernatants by incubation with nativeor formalin-treated K562 or SMMC. 7721 cells at 37 ° C or 4 ° C LT activity can be adsorbed with L929 cells. Beforefore, LR appears to be cytotoxic / cytostatic factor that is distinctfrom LT, IFN and NKCF.