The cell surface antigen phenotype of circulating and skin malignant T-cells in patients with cutaneous T-cell lymphoma were studied. The mature T-cell antigen phenotype of the malignant T-cells was identical for circulating and skin malignant T-cells. In contrast, skin malignant T-cells expressed the immature human T-cell antigen Thy-1, surface membrane transferrin receptors, and Ia-like determinants while circulating malignant T-cells did not express these antigens. Skin infiltrating T-cells in benign dermatoses also expressed Thy-1 and Ia-like determinants. These observations support the notion that the skin is a major site of extrathymic T-cell differentiation and may promote phenotypic changes in circulating T-cells that home to skin.