Subcellular distribution of cathepsin B following subfractionation of the kidney cortex mitochondrial/lysosomal fraction by rate sedimentation indicates that this enzyme is mainly associated with the large, fast sedimenting lysosomes (protein droplets). A small proportion of cathepsin B is also present in the small lysosomes which cosediment with mitochondria, peroxisomes, and brush border and other large membrane vesicles. Amongst this broad spectrum of small lysosomes the distribution of cathepsin B, together with other acid hydrolases is associated with the more rapidly sedimenting lysosomes whilst cathepsin D differs in being associated with the slowest sedimenting lysosomes. Equilibrium banding in sucrose gradients shows the large lysosomes band at a density of 1.235 g/ml and that the small lysosomes have two distinct populations at densities 1.20 and 1.235 g/ml. Cathepsin B (and also cathepsin D and acid ribonuclease) appears to be associated only with lysosomes of high density. The various other acid hydrolases assayed are found in all the lysosomal populations. Small and large lysosomes of high density are very rich in a number of proteinases and therefore most probably represent lysosomal populations involved in the catabolism of proteins taken up from the glomerular filtrate.