We assessed natural killer (NK) cell activity against cultured melanoma cells using a novel method, with observations on the comparative effects of interferons (IFNs) in NK-stimulating and anti-proliferative assays. Since the tumour cells tested were adherent, a semi-automated colorimetric MTT dye reduction assay was developed to assess NK activity. The three adherent human melanoma cell lines Sk-Mel-28, MM418 and MM96 were shown to be suitable targets for determining NK activity. Also, these were representative of the range of sensitivities of melanoma cells to the anti-proliferative action of type 1 IFNs. The dose-dependent stimulation of NK activity by type 1 IFNs was confirmed in this alternative assay system. IFN-alpha 2b and IFN-beta ser had equivalent stimulatory activities, and IFN-alpha 4 proved less effective, as with assays using the classical K562 system. Augmented NK cytotoxicity did not correlate with anti-proliferative effects of IFN. In anti-proliferative assays, the hierarchy of activity is IFN-beta greater than IFN-alpha 2 greater than IFN-alpha 4, whereas, in the NK augmentation assay IFN-beta and IFN-alpha 2 were of equivalent activity. Interestingly, MM96 was the cell line most resistant to the direct anti-proliferative action of IFN, yet it was the most susceptible of the melanoma cell lines to cytotoxicity by NK cells, whether stimulated or unstimulated by IFN.