'Mature' human natural killer cell activity could be enriched by adsorption-elution with fetal fibroblast as adsorbents against normal allogeneic skin fibroblasts, but not against autologous targets. However, when the natural killer activity was augmented by contact with tumour cells (HeLa), autologous and allogeneic skin fibroblast targets were killed without preference. Adsorption-elution with augmenting target cells as adsorbents resulted in an efficient enrichment of NK activity showing non-discriminate cytotoxicity. Morphologically, this was associated with an enrichment of large granular lymphocytes previously shown to be responsible for human NK activity. We conclude that the NK activity of human 'mature' NK cells shows a relative autologous exemption, whereas 'pre-NK' cells augmented to full activity in vitro are non-discriminative.