Plasminogen activator (PA) is a serine protease with a proven medical application as a fibrinolytic drug. PA is a normal constituent of animal tissues and is found in low concentration 1-10 U/ml in the supernatant of various cells in culture. We found that certain strains of untransformed human diploid fibroblasts can be adapted to produce PA at levels of 100 U/ml or more by culture with specific combinations of serum and coated substrates. Once the cells are adapted, they continue to produce the enzyme in a medium devoid of serum in which cell proliferation is minimal. The time course of enzyme production revealed that its formation is controlled by negative feedback and stops when its level in the medium reaches a critical concentration. However on removal of the enzyme by continuously replacing the medium, the cells continue to produce enzyme at a constant rate for a period of several weeks. Possible biotechnological configurations for large scale production of this enzyme are now under study. The PA purified from the spent medium of these cells was identified as tissue type activator similar to the melanoma type PA.