Recently we described a new epidermal serine proteinase inhibitor, skin-derived antileukoproteinase (SKALP), also known as elafin. SKALP/elafin was found to be absent in normal human epidermis, but can be induced in vitro and in vivo under hyperproliferative conditions. Here we studied the expression of SKALP/elafin in several types of epidermal tumors (basal cell carcinoma, squamous cell carcinoma, Bowen's disease, actinic keratosis, and keratoacanthoma). Using immunohistochemical staining SKALP/elafin appeared to be differentially expressed in these tumors. Functional measurements of anti-proteinase activity, and Western blotting of tumor extracts confirmed our findings at the histological level. In well differentiated squamous cell carcinoma, SKALP/elafin messenger RNA was demonstrated by non-radioactive in situ hybridization. We conclude that SKALP/elafin is a marker for abnormal or disturbed squamous differentiation. A possible role of SKALP/elafin in the control of tumor cell invasion is discussed.