The liver metastasis formation of two human melanoma cell lines were compared in male and female SCID mice. The intrasplenic injection of both tumour lines resulted in a significantly higher number of liver metastases in male than in female mice; the incidence and weight of spleen tumours, as well as the incidence of metastases were similar. Both melanoma cell lines bound fluorescent oestradiol, progesterone and testosterone conjugates, and proved to be positive for oestrogen receptor-related protein by immunocytochemistry. These observations support the view that endocrine factors influence the progression of human melanomas. This SCID mouse model could be useful in studying the effects of hormonal manipulations on human melanoma metastases.