The significance of Leu 8 negative T cells in lymphoid skin infiltrates: malignant transformation, selective homing or T-cell activation? The expression of Leu 8 was studied on skin biopsies from a large group of patients with benign and malignant skin disorders and correlated with the expression of T-cell differentiation antigens and activation markers. The effect of in vitro stimulation of peripheral blood T cells and T-cell subsets on the expression of Leu 8 antigen was also determined. In all the skin diseases studied an inverse relationship was found between the proportions of cells expressing Leu 8 and HLA-DR. A deficiency of Leu 8 positive cells was not specific for mycosis fungoides, but was also found in several reactive dermatoses. Stimulation of peripheral blood cells with phytohaemagglutinin (PHA), concanavalin A (Con A), and anti-CD3-PMA resulted in a considerable decrease of Leu 8 antigen expression on day 3 in both CD4+ and CD8+ T cells. These data suggest that the low proportion of Leu 8+ T cells in mycosis fungoides and several reactive skin disorders is not related to malignant transformation or selective homing of Leu 8- T cells, but probably results from local T-cell activation. 