UM4D4 (CDw60), the surface molecule of a novel antigen- independent T-cell activation pathway, was found to be highly expressed on lesional psoriatic T cells. To examine whether UM4D4 represents a T-cell activation pathway for psoriatic T cells, a T-cell line was initiated from an acute skin lesion and cloned by limiting dilution. Clonality was verified by analysis of T-cell receptor gene rearrangement. All T-cell clones tested, whether CD4⁺2H4⁺CD8^-,CD4⁺2H4^-CD8^-, or CD4^-CD8⁺CD11b^-, expressed UM4D4 and were activated by the monoclonal antibody anti-UM4D4. Lesional psoriatic T-cell clones were heterogeneous in the degree of anti-UM4D4-induced proliferation and in their production of IL-2 and gamma-interferon. Lymphokines released by anti-UM4D4 activation were capable of inducing ICAM-1 and HLA-DR expression on cultured normal keratinocytes. Thus, the high expression of UM4D4 on T-cells in psoriatic skin provides an alternative mechanism for T-cell activation that may be operative in the psoriatic lesional milieu. Indeed, activation of lesional T-cells through the UM4D4 molecule resulted in release of lymphokines that directly induced keratinocytes to express a phenotype displayed in psoriatic skin lesions.