A population of cells showing the surface phenotype of Langerhans cells (LCs) was identified in the dermal infiltrates of cutaneous T cell lymphoma (CTCL). Peroxidase-conjugated OKT6, a monoclonal antibody reactive with epidermal LCs, was used to directly label frozen tissue sections of diseased skin from twenty-three patients with CTCL, two patients with secondary cutaneous involvement by a B cell lymphoma, and three patients with lymphocytoma cutis. OKT6-reactive cells represented a significant although minor population in the dermal infiltrate of twenty-two of the twenty-three CTCL biopsies, accounting for up to 5% of the cells. Double-labeling studies revealed that the OKT6-positive cells also exhibited Ia but not T cell antigens. Since OKT6-reactive cells were not found in either the B cell lymphomas or lymphocytorna cutis, their presence in a malignant infiltrate is suggestive of a T cell neoplasm.