Cultures of keratinocytes from uninvolved psoriatic skin have been shown to exhibit increased DNA synthesis. Since epidermal cell-derived thymocyte-activating factor (ETAF) can stimulate the proliferation of human keratinocytes in vitro, we have compared the formation of ETAF in cultured keratinocytes from normal and uninvolved psoriatic skin. Trypsinized epidermal cells were plated at nonconfluent concentrations in dishes coated with a collagen type I gel. Normal keratinocyte cultures spontaneously released ETAF into the supernatant from day 1, reaching a maximum level (mean 124 +/- 4 units/micrograms DNA) on day 7, just before the cultures became completely confluent (days 9-12). The ETAF release then gradually decreased until a plateau (mean 16 +/- 7 units/micrograms DNA) was reached on day 18. In normal keratinocyte cultures the incorporation of [3H]thymidine into DNA changed in parallel with the ETAF release. Psoriatic keratinocytes had normal rates of ETAF release and of DNA synthesis until the time of culture confluency. However, when complete confluency was obtained, psoriatic keratinocytes continued to synthesize DNA at high levels (increased by 146% on day 15) while their ETAF release declined. Analysis of intracellular ETAF showed a significant correlation with the extracellular ETAF in cultures from both normal (r = 0.77, n = 10, p less than 0.02) and uninvolved psoriatic skin (r = 0.76, n = 12, p less than 0.01). These results indicate a relation between ETAF formation and DNA synthesis of normal keratinocyte cultures. However, factors other than ETAF appear to be responsible for the elevated DNA synthesis of confluent keratinocyte cultures from uninvolved psoriatic skin.