To initiate investigations into the effects of enkephalins on immune function in cancer patients, the effect of methionine-enkephalin and leucine-enkephalin on natural killer (NK) cell activity in isolated peripheral blood lymphocytes from cancer patients was investigated. Incubation of lymphocytes with either enkephalin resulted in significant increases in NK cell activity. At effector:target cell ratios of 100:1, 33:1 and 11:1 leucine-enkephalin significantly (p less than 0.05) enhanced NK activity at dilutions of 10(-6), 10(-8), 10(-10), and 10(-14) mg/ml. Similar results were obtained with methionine-enkephalin with the exception that the 10(-6) dilution gave insignificant changes at both the 33:1 and 11:1 cell ratios. The results indicate a difference in dose response to both enkephalins between lymphocytes from cancer patients and normal volunteers.