UV irradiated cells release into the culture medium factors that induce, when given to nonirradiated cells, the transcription of several UV-inducible genes (collagenase I, human immunodeficiency virus type 1, metallothionein IIA). We identify here the active factors released from UV-treated HeLa cells, as interleukin 1 alpha and basic fibroblast growth factor. UV irradiation leads to increased mRNA levels for both factors and to their enhanced synthesis. Experiments with the drug suramin, which inhibits growth factor-growth factor receptor interactions and with antibodies directed against interleukin 1 alpha and basic fibroblast growth factor, suggest that growth factors do not only transduce the UV-induced signal to nonirradiated cells but act on the producer cell thus establishing an obligatory growth factor loop for at least part of the UV response.