In malignant melanoma, melanocyte-stimulating hormone (alpha-MSH) has been found to influence the cellular metabolism of melanoma cells (c-AMP production, protein and RNA synthesis, and tyrosinase activation). In some publications elevated alpha-MSH levels have been described in melanoma patients. In the present study we used a commercially available radioimmunoassay to examine the alpha-MSH levels in patients with malignant melanoma and a control group consisting of apparently healthy volunteers (laboratory assistants) and dermatological patients without malignant tumours. The plasma alpha-MSH levels were (mean +/- SD) 12.2 +/- 12.9 for 37 melanoma patients (17 female, 20 male) and 7.9 +/- 3.5 pmol/l for 38 control persons (18 female, 20 male). The difference is significant according to the distribution-free U-test of Mann and Whitney. In 13 (35%) of the melanoma patients values were above the normal range defined by the 95.5% confidence limit. alpha-MSH cannot be classified as a typical tumour marker. Nonetheless, in our opinion alpha-MSH levels may be useful in monitoring melanoma patients with reference to prognosis and follow up during and after therapy.