Involucrin is a protein precursor of the epidermal cornified envelope. Although expression of the human protein has been documented extensively, studies in the mouse have been hampered by a shortage of good antibodies. We describe the production of recombinant mouse involucrin and preparation of rabbit antisera to the protein that work well by immunohistochemistry and Western blotting. We confirm that in normal mouse epidermis the onset of involucrin expression is in the upper spinous layers and inner root sheath of the hair follicle. Involucrin was also detected in the differentiating epithelial cells of normal tongue, oesophagus and bladder. Involucrin was expressed in a subpopulation of mouse keratinocytes cultured in standard or low calcium medium and the proportion of involucrin-positive cells increased during suspension-induced terminal differentiation. Western blotting of keratinocytes from several inbred mouse strains revealed a remarkable heterogeneity in the electrophoretic mobility of involucrin, reflecting inter-strain variation in the number of tandem repeats in the protein. In the hyperproliferative epidermis of healing wounds involucrin was expressed in most of the suprabasal layers. In epidermal papillomas and carcinomas involucrin expression correlated well with degree of histological differentiation. The sites of expression of the mouse protein were thus the same as those previously reported for human involucrin. With the development of the new antibodies we anticipate that involucrin will become as widely used a marker of keratinocyte differentiation in the mouse as it is in the human.