The human skin equivalent (HSE) provides a convenient model system for studying the cellular responses of basal keratinocytes to UV irradiation. HSEs, constructed by overlaying a collagen-fibroblast matrix with epidermal cells, were raised to an air-liquid interface to promote epidermal differentiation. HSEs were exposed to ultraviolet radiation from a 500-W Hot Quartz Hanovia therapeutic sunlamp, at a total dose of 100 J/m2. The HSEs were then frozen every 4 h over a 48-h period and cryosectioned. For each time period, the expression of beta1 integrin and cyclin E, p53, or Bcl-2 were quantified using dual immunolocalization. Basal cells expressing beta1 integrin were divided into two subpopulations, denoted beta1(high) or beta1(low). The proportion of beta1(high) keratinocytes expressing Bcl-2 and cyclin E increased significantly 4 and 8 h, respectively, after exposure to UV; during the subsequent 16 h, this basal cell subpopulation expressed p53. By contrast, significant numbers of beta1(low) basal keratinocytes expressed p53, but not Bcl-2. These results suggest that beta1(high) and beta1(low) populations of basal epidermal cells in HSEs respond differently to UV irradiation.