The expression of Thy-1 was examined on fresh frozen sections of melanoma and control skin lesions using a monoclonal antibody to Thy-1 and immunoperoxidase-labelled second antibodies. Thy-1 was detected on 12 of 19 primary melanomas but on only 1 of 23 melanoma metastases in lymph nodes, skin or brain. Expression of Thy-1 did not appear related to other known histological features of primary melanoma. Thy-1 was also expressed on 2 of 7 naevi but not on normal melanocytes or carcinoma of squamous cells, basal cells and sweat glands of the skin. Some lymphocytes below primary melanomas were Thy-1 positive. These results suggest that Thy-1 may be a marker of melanocytes at certain stages of differentiation analogous to its expression on cell lineages in the haemopoietic system. The relative paucity of Thy-1 on metastases may indicate that its expression on melanoma cells has prognostic significance for behaviour of the tumour in its host.