Normal human proliferative keratinocytes can be serially cultured in serum-free medium. The medium and culture conditions that have been developed support the growth of these cells in the absence of a feeder layer and in the absence of contaminating fibroblasts. Using this system, the nutritional and hormonal factors that affect the growth and differentiation of these cells have been studied in the absence of undefined supplements. The data suggest that the control of proliferation and differentiation in keratinocytes may take place by unique mechanisms. The defined serum-free conditions that have been developed provide an excellent system for the study of nutritional disorders that affect human epidermal cells. In addition, the culture system can be used to propagate large numbers of cells for use in autografts for cases of severe burns.