Keratinocyte/leucocyte interactions have become an area of intense investigations in the last decade. However, few convenient in vitro models are available at present. We have therefore designed a novel in vitro system for autologous human keratinocyte/leucocyte co-culture. Non-invasive epidermal cell sampling was achieved by using outer root sheath cells from hair follicles. After one passage, pure keratinocyte cultures (no Langerhans cells or melanocytes) were obtained. Co-culture experiments were performed on a Transwell system: keratinocytes were grown on the porous cupula, and then laid on to wells containing leucocytes. Alternatively, leucocytes can be added to the cupula when contact interactions between the two cell types are to be investigated. Using this system, we demonstrated that Phaesolus vulgaris phytohaemagglutinin-activated T lymphocytes (with 10% monocytes) in the lower compartment induced intercellular adhesion molecule 1 (ICAM-1) and HLA-DR expression, and inhibited methyl-3H-thymidine incorporation in normal human autologous keratinocytes cultured on the cupula. These changes were mediated by soluble factors (no cell contacts between keratinocytes and leucocytes), and required lymphocyte activation. This is the first direct in vitro evidence for leucocyte-induced ICAM-1 and HLA-DR expression on keratinocytes. This system is a potential tool for the study of keratinocyte/leucocyte interactions.(ABSTRACT TRUNCATED AT 250 WORDS)