We have screened a cDNA library for gene sequences that are regulated by platelet-derived growth factor (PDGF) in BALB/c-3T3 cells. Of 8000 clones screened, less than 14 independent PDGF-inducible sequences were found. Two of these (KC and JE) were studied in detail. By hybrid-selection and translation the KC and JE mRNAs encode 10,000 and 19,000 dalton polypeptides, respectively. In the absence of PDGF, the JE and KC sequences correspond to low abundance mRNAs. One hour after addition of PDGF their abundance level can be increased 10- to 20-fold. Within 4 hr, a 60-fold induction of JE can be attained. Nanogram per ml quantities of pure PDGF regulate these sequences whereas microgram/ml quantities of chemically unrelated mitogens (EGF, insulin, or platelet-poor plasma) have either a weak or an undetectable effect. Inhibitors of protein synthesis block the progression of quiescent 3T3 cells through G1 into S phase; however these drugs do not block the induction of KC and JE by PDGF. This result indicates that these sequences correspond to "early genes" which are not induced as a consequence of cell growth, but rather are directly regulated by PDGF.