The effect of exogenous Namalva interferon (IF) on the natural killer (NK) cell activity of human blood lymphocytes was examined against 5 target cell lines (K562, CCRF/CEM, Molt 4, Raji and Bri8) using the 51Cr-release assay. Addition of IF to the test significantly increased the cytotoxicity, though not as much as when effector cells were treated with IF before the test. Augmentation of cytotoxicity was evident after only 1 h pretreatment and was maximal by 6 h. The rate of lysis of susceptible targets by IF-treated effectors markedly exceeded that by their untreated counterparts. Separation of lymphocyte subpopulations (by SRBC-rosette sedimentation and nylon-fibre column filtration) demonstrated that the activities of IF-stimulated and unstimulated cells were similarly distributed, suggesting that the major effect of IF is enhancement of the activity of pre-existing NK cells rather than generation of new populations of effectors. Target cell lines with high and low susceptibility to NK cells showed increased cytotoxicity by IF-treated effector cells. These findings may be relevant to the current discussion of the role of NK cells in immunosurveillance against neoplasia.