Cultured epithelial cells are used to generate extensive patches of autologous skin equivalent for patients with burns or wounds and to investigate the growth and differentiation of epithelia in vitro. We have undertaken a comprehensive study of the morphological and molecular events that occur during culturing of human foreskin keratinocytes at the liquid-air interface on a dermal equivalent consisting of a collagen matrix containing fibroblasts. Using radioactively labeled RNA probes for mRNAs and monoclonal antibodies for proteins, we found that the expression of a comprehensive set of differentiation stage-specific genes was affected by the type of fibroblasts included in the matrix as well as by the age of the culture. The expression of these genes was not always coordinated and could not be predicted from the histological appearance of the stratified epithelium. Surprisingly, the mouse fibroblasts promoted epithelial differentiation much more closely resembling foreskin than did the homologous primary foreskin fibroblasts.