BACKGROUND: During terminal differentiation, basal keratinocytes lose gradually contact with the basement membrane, a process accompanied by the progressive functional down-regulation and loss of integrin expression. Understanding the molecular nature of this complex mechanism will eventually lead to insight into the pathogenesis of differentiation disorders of the epidermis, e.g. psoriasis. OBJECTIVE: The monoclonal antibody 8D9 against the very late antigen 5 (VLA-5) integrin subunit was used to study the expression and down-regulation of this protein in several experimental paradigms of keratinocyte differentiation. METHODS: Primary cultures of human keratinocytes were prepared and used as such, or after induction of terminal differentiation with methylcellulose and/or calcium. Expression of the 8D9 epitope was analyzed using immunoblotting, protein chemistry and immunocytochemistry on cultured cells and on skin biopsies from control and psoriatic patients. RESULTS AND CONCLUSION: The monoclonal antibody 8D9 reacts with the alpha 5-subunit of human VLA-5 integrin and with a 68-kD antigen that is strongly expressed in differentiating keratinocytes in vitro and in the cornified layers of human skin in vivo. Psoriatic skin showed additional immunoreactivity in the upper spinous and granular layers. Based on indirect immunological and chemical evidence we suggest that the 68-kD protein is an amino-terminal degradation product of the alpha 5-subunit, which provides a new and interesting marker of differentiating keratinocytes.