Involucrin, the major protein precursor of the cornified envelope, is expressed during terminal differentiation of human keratinocytes, both in vivo and in vitro. In epidermis, the onset of synthesis is several layers above the basal layer, but in stratified cultures of keratinocytes on tissue culture plastic involucrin synthesis begins in the first suprabasal layer. To investigate the reason for this premature expression, the distribution of involucrin was studied in epidermis from different body sites, in organotypic cultures and in transplants of keratinocytes onto nude mice. We found that premature expression was not associated with poor morphological differentiation, because involucrin synthesis began immediately above the basal layer even when distinct basal, spinous, granular and cornified layers were formed in organotypic cultures recombined with dermis. The site of involucrin expression in culture did not depend on the number of cornified layers present. The only conditions which resulted in an upward shift in the site of synthesis were in 3-week old transplants on nude mice. We conclude that the site of onset of involucrin synthesis is not determined by the degree of morphological differentiation of the tissue, and discuss other factors which may be involved.