Clinically uninvolved psoriatic epidermis shows increased DNA synthesis in vivo. We have studied the DNA synthesis of cultured keratinocytes from uninvolved psoriatic skin. Trypsinized epidermal cells were plated on plastic dishes pre-coated with bovine collagen type I. In initial studies, normal human serum was found to be superior to fetal bovine in supporting the growth of human epidermal keratinocytes. Furthermore, keratinocyte cultures established in the presence of normal human serum produced large keratin proteins (68,000 daltons) indicating that the terminal steps in cell differentiation can occur in vitro. In subsequent experiments keratinocyte cultures were grown in medium supplemented with 10% normal human serum. Confluent cultures of keratinocytes from uninvolved psoriatic epidermis had an increased DNA synthesis determined both as the incorporation of [3H]thymidine and as the autoradiographic labelling index. The DNA synthesis of both normal and psoriatic keratinocyte cultures increased in response to incubation in medium with 10% psoriatic serum. The ability of keratinocytes from uninvolved psoriatic epidermis to maintain an increased DNA synthesis suggests the presence of an inherent defect within the population of epidermal keratinocytes in psoriasis. Such a culture system can be used as an in vitro model for the study of psoriasis.