The results of several investigations proved that, in special circumstances, human keratinocytes (HKs) synthesize and express cell surface moieties characteristic of effector and/or accessory cells of the immune system, such as CD16, CD36, HLA-DR, and intercellular adhesion molecule-1 (CD54), which are all detectable on the surfaces of macrophages. In the present study, skin biopsies from healthy volunteers, from positive tuberculin skin tests, and from patients with acute urticaria (AU), lichen planus (LP), psoriasis vulgaris (PV), mycosis fungoides (MF), and purpura pigmentosa chronica (PPC) were investigated by means of a multistep immunoperoxidase method to examine the reactivity of the HKs with a panel of monoclonal antibodies (MABs) characteristic of monocyte/macrophage cell lines. In biopsies obtained from positive tuberculin tests and from clinically involved skin of patients with LP, PV, MF, or PPC, a multifocal, positive peroxidase reaction was observed on the membranes of HKs of the basal and suprabasal cell layers when the MABs OKM13 (CD13), OKM14 (CD14), and Dako-Macrophage (CD68) were used. In contrast, specific staining of the HKs was not observed with the same antibodies in the biopsies of healthy volunteers or of patients with AU or in the uninvolved skin specimens obtained from the other patients. The HKs of PV, LP, MF, PPC, and AU patients and those of the healthy subjects all failed to give positive reactions when MABs against CD11b, CD15, or CD33 were used. The published data supplement the known surface characteristics of HKs, reflecting their stage of activation and differentiation.