Skin biopsies from 49 patients with 21 different dermatoses and biopsies of normal skin were investigated with regard to the reactivity of keratinocytes with the monoclonal antimonocytic antibody anti-Leu M2 using the avidin-biotin-peroxidase complex method. In all excisates, a cytoplasmic reaction of keratinocytes was seen which was regularly weaker in the basal layers of the epidermis than in the upper layers. Marked differences in the labelling intensity were shown between the individual dermatoses. Keratinocytes have now been shown to react with several monoclonal antibodies directed against monocytic antigens; therefore the functional significance of this should be discussed.