SINCE Astrup and Permin1 reported that animal tissue cells contain an activator of plasminogen, various authors have confirmed their results, and comparisons of the activator content of different human2 and animal tissues3,4 have been published. Most authors have used whole tissue or aqueous tissue extracts and have estimated activity by the fibrin plate method of Astrup and Mullertz5. Tagnon and Peterman6, and Tagnon and Palade7 fractionated rat lung tissue by differential centrifugation following the method of Claude8 and found activity in the particulate (notably ‘microsome’) fraction. Lewis and Ferguson9, using Claude's method of fractionation of tissues and a fibrin clot lysis time method of assay, compared the different tissue fractions of the dog and compared the activity of mouse, cat, dog, human and rat lung fractions. They found that activity lay particularly in the ‘large granule’ (2,700 g) and microsomal (48,000 g) fractions, that the activator was relatively heat labile and showed no real species specificity.