Reactivity for Ki-1 antibody was studied in 145 patients with a large variety of cutaneous disorders. The antigen was consistently expressed and by a high proportion of tumour cells in infiltrates in which atypical cells revealed a 'histiocytic' appearance, i.e. lymphomatoid papulosis (LP), T-immunoblastic lymphoma with the characteristic of true histiocytic lymphoma, Hodgkin's disease, T-blast cell proliferation with giant multivesicular bodies, concurrent LP and mycosis fungoides (MF), and two cases of MF. Ki-1+ cells with the usual morphology of atypical T-cells formed a major component in 2 other cases of MF, and a minor component in 7 other cases of MF. A possible non-neoplastic counterpart was found in small to medium-sized Ki-1+ cells, including blast cells, which occurred occasionally in the T-cell infiltrates of eczema, actinic reticuloid, lichen planus and pityriasis lichenoides. Small Ki-1+ cells which were observed in the reactive B/T cell component of lymphocytoma cutis but also in similar components occurring occasionally in non-epidermotropic cutaneous T-cell lymphoma, and malignant B-cell lymphomas, might be analogous to the Ki-1+ cells in normal lymphoid tissue.