Contact with various cell-line targets increases the natural killer (NK) activity of human lymphocytes. Supernatants of such 20 h co-cultures augment the NK activity of virgin lymphocyte populations, and the augmenting factor penetrates 0.2 micrometer Millipore filters. The supernatants also contain interferon, and partially purified human leucocyte interferon increases NK activity when added to 20 h assays with 51Cr-labelled K-562 target cells. Potent anti-interferon antiserum added to the co-cultures inhibits the target-cell-induced augmentation phenomenon. Both the target-cell contact and interferon-induced augmentation affect a population of human lymphocytes, from which the 'mature' NK cells have been removed by adsorption-elution using fetal fibroblasts as adsorbents. The activity of 'mature' NK cells is not enhanced, and we conclude that the augmentation is mediated by recruitment of 'pre-NK' cells.