An inverse correlation between the nm23 RNA level and tumour progression of melanocytes has been reported. To elucidate whether the expression of nm23 gene product in malignant melanoma is also inversely correlated with metastatic potential, conventional prognostic parameters or the tumour suppressor protein p53, immunohisto-chemical studies using a monoclonal antibody against nm23-H1 protein were performed on 138 benign and malignant melanocytic tumours. The expression of nm23 protein was compared with that of p53 protein and conventional clinicopathological prognostic factors. The nm23 protein level in benign melanocytes and metastatic melanoma cells was also studied by Western blot analysis. No significant difference regarding the protein was observed between naevi and melanomas, either at histological or protein levels. The expression correlated with local recurrence within 1 year after surgery, level of invasion and tumour thickness, but no parallels were observed between the nm23 and p53 proteins, suggesting that gene is regulated by independent mechanisms, although located on the same chromosome. There was no inverse correlation between the nm23 protein and melanoma metastasis which suggested that the nm23 protein does not appear to be lost during melanoma metastasis.