On the basis of cellular morphology, a subline of mouse sarcoma virus-infected 3T3 cells was selected which released a 48 000-dalton plasminogen activator at an approx. 40-fold higher rate than those of the parent line, and which continued to do so for several months when the cells were maintained in serum-free culture medium. Culture medium (3.5 l) containing 0.6 mg plasminogen activator per l was used to purify 620 micrograms of the enzyme 130-fold with a yield of 32% by affinity chromatography followed by anion exchange chromatography and gel filtration. Crucial for the yield was the use of a non-ionic detergent and of inhibitors of proteolysis to prevent adsorption and degradation, respectively. The purified enzyme was homogeneous as evaluated by SDS-polyacrylamide gel electrophoresis and had an isoelectric point of pH 9.2. The purified enzyme showed characteristics of a trypsin-like serine protease (labeling with [3H]diisopropylphosphorofluoridate which was prevented by p-nitrophenyl-p'-guanidinobenzoate) and converted the single chain of human plasminogen into two chains of plasmin with electrophoretic mobilities identical to those of the chains formed by non-purified enzyme and by human urokinase. In the absence of inhibitors, solutions of purified enzyme were stable for 24 h at 4 degrees C at pH 3-9.