Incubation of kidney lysosomes at 37° results in a graded release of lysosomal enzymes. The release of enzyme occurs in two stages. First the enzymes become available to the substrate but remain sedimentable. Later the amount of soluble enzyme increases and eventually is almost equal to that of the available enzyme. Morphological studies of lysosomes showed that during the process involving increasing availability of enzymes, the lysosomes remained intact. Release of the soluble enzymes was characterized ultrastructurally by a complete loss of the electron-opaque matrix contained within the lysosomal membrane. The increased release of soluble enzymes was concomitant with an increase in the number of individual lysosomes showing complete loss of contents, rather than a gradual loss or dilution of matrix density. Lysosomes which had lost their electron-opaque contents retained their outer membrane intact and were seen to contain numerous internal membranes and small vesicles.