Induction of synthesis of the protease plasminogen activator (PA) by hormones, oncogenic viruses and tumor promoters occurs at the transcription level. A novel bioassay for PA messenger RNA was developed to study the regulation of PA synthesis and the genetic elements involved in it. Poly(A)-containing RNA from HEp-3, a PA-rich tumor of human origin, was found to direct the synthesis of a new proteolytic activity when microinjected into Xenopus oocytes. Newly synthesized protease can be detected within a few hours after microinjection of minute quantities of unfractionated mRNA. The new enzymatic activity is indistinguishable from human PA: it is absolutely dependent on human plasminogen; it is neutralized by serum raised against urokinase, the human urinary PA; and it comigrates with urokinase and HEp-3 PA in gel electrophoresis, exhibiting a molecular weight of 60,000.