1. A rat kidney lysosomal fraction was prepared by the method of Maunsbach (1966) and characterized by its content of representative marker enzymes for lysosomes, mitochondria, peroxisomes and endoplasmic reticulum. 2. It was shown that both pH-dependent and progesterone-induced lysis lead to a decrease in the E520 of suspensions of this preparation. This decrease parallels quantitatively and temporally the release of soluble acid phosphatase. 3. It is suggested that E520 measurements are a valid method for the continuous measurement of changes in lysosomal integrity. 4. As an example, results are included which demonstrate the ability of Zn2+ to stabilize lysosomes against spontaneous and progesterone-induced lysis.
