Malignant melanoma is one of the fulminant skin cancers. The 5-year survival of patients with stage III (N0, N1) malignant melanoma treated with multi-agent chemoimmunotherapy, including natural interferon-beta (nIFNbeta), was found in our department to be better than that of patients treated with other forms of therapy. In order to study the effects of nIFNbeta on melanoma, the growth inhibition effect of nIFNbeta was assessed in vitro using the melanoma cell lines, MM8.1, MM28, MM33.1, Bowes and A375-2. The growth of these cell lines was inhibited by nIFNbeta. Incorporation of [3H]thymidine and [3H]uridine was also inhibited by nIFNbeta in a dose-dependent manner. Apoptosis was demonstrated using the TUNEL method in melanoma cell lines cocultured with nIFNbeta. Results showed that nIFNbeta had direct killer activity on melanoma cell lines.