The effects of the PDGF isoforms AA, AB and BB have been studied on the proliferation of normal rat kidney cells, a non-tumorigenic fibroblast cell line which contains both typeaand type p PDGF receptors. On monolayer cells made quiescent by serum deprivation, PDGF-AA is a relatively poor mitogen compared to PDGF-AB and PDGF-BB. When these cells are made density-arrested following continuous incubation with epidermal growth factor, however, they can be restimulated to proliferate by all three PDGF isoforms with similar activity when added at sufficiently high concentration, resulting in phenotypic cellular transformation. Binding of radiolabelled isoforms to confluent NRK monolayers obeys the predictions of an induced receptor dimidiation model, and increases in the order AA