1. On subcellular fractionation of rabbit kidney by differential and density-gradient centrifugation, a high proportion of the tissue activator of plasminogen activity was found to be particulate and displayed sedimentation properties associated with the lysosome-rich fraction as judged biochemically by the acid-phosphatase activity. 2. The activator activity is closely associated with a latent protease whose activity is enhanced in the presence of Triton X-100 or sodium deoxycholate in the neutral pH range. Besides hydrolysing casein this protease is also capable of attacking fibrinogen at pH7.4. 3. The pH optimum for activator activity and its inhibition by in-hexanoic acid (in-aminocaproic acid) point to its possible similarity to urokinase, an activator of plasminogen present in the urine of most mammals.