Dermatopathologists observe mononuclear leukocytes in close apposition to keratinocytes (KCs) in graft versus host disease and in other lymphocyte-mediated skin diseases, such as lichen planus, erythema multiforme, and lupus erythematosus. Since the KCs are Class II histo-compatibility antigen (HLA-DR) positive in these diseases (indicating local production of gamma interferon, IFN-γ, by activated T-cells), we sought to determine whether IFN-γ treatment of KCs would influence the ability of allogeneic peripheral blood mononuclear leukocytes (PBMLs) to adhere to cultured KCs in vitro. The adherence of PBMLs to KC monolayers was determined by the three following methods: (a) methanol fixation of the washed KCs (after PBML incubation), followed by hematoxylin-eosin staining and direct counting of adherent PBMLs; (b) fluorescein isothiocyanate (FITC) labeling of PBML, followed by measuring the amount of FITC-PBML bound to KCs after washing either by direct visualization with a fluorescence microscope: or by (c) quantitative fluorescence spectroscopy following lysis of the adherent cells. While untreated KCs bound allogeneic PBMLs minimally 15–120 min at 37°C, pretreatment of the KCs with IFN-γ (300 U/ml, 3 days) produced significantly increased binding of the PBMLs by approximately fivefold. By contrast, IFN-α and IFN-β (103 U/ml) had no effect. Also, despite the induction of HLA-DR on cultured human fibroblasts, no increased binding of PBMLs after IFN-γ treatment was observed. The selective ability of IFN-γ to produce a marked increase in adherence between KCs and PBMLs suggests a new role for IFN-γ in the immunobiology of the skin.