The cytotoxic activity of natural killer cells and the intensity of conjugate formation are studied in vitro in the natural cytotoxicity reaction against H-3-uridine-labeled human erythromyeloleukotic cells K-562 in the presence of fibronectin, gamma-globulin, and fibronectin/gamma-globulin combination. It is demonstrated that fibronectin does not change natural cytotoxicity, gamma-globulin increases the activity of human natural killer cells, and the fibronectin - gamma-globulin combination increases both the intensity of conjugate formation and the cytotoxic activity of natural killer cells.