Summary When the concentration of SK was carefully controlled, a strong covariance relationship was found between the plasminogen and proactivator concentration in 12 normal human plasma samples. It is suggested that lack of such control had led earlier investigators to the erroneous conclusion that they had demonstrated the presence of a proactivator other than plasminogen or plasmin in human plasma. The euglobulin precipitate at pH 5.3 contained virtually all of the plasminogen present in the original plasma.