The effect of synthetic opioid peptide analogue dalargin on the natural cytotoxicity of lymphocytes from the peripheral human blood has been studied in respect to target cells K-562. Natural cytotoxicity was determined by assessing the cytotoxicity of individual effector cells in agarose. It has been shown that preincubation of lymphocytes with 10(-14)--10(-8) M of dalargin enhances the binding of NK-cells with targets and the formation of conjugates with killed targets and increases the percentage of active NK-cells. In some cases bimodal kinetics of a stimulating dalargin effect on the natural cytotoxicity has been observed, with two peaks of the peptide activity being in the range of 10(14)--10(-12) and 10(-10)--10(-6) M. Naloxone at a concentration of 10(-6) M acted as an agonist of opioid receptors, enhancing the formation of effector-target conjugates and NK-cell lysis abilities. A stimulating effect of dalargin on the natural cytotoxicity was more marked than that of recombinant human alpha-interferon. The possible use of dalargin in pathological conditions characterized by disturbed NK-cell function is discussed.