Commercially available Wistar rats are genetically heterogeneous with respect to acid beta-D-glucuronidase (EC 3.2.1.31) in liver tissue: Of 43 rats studied, 27 animals exhibited only approximately 40% catalytic activity of the remaining group. Analysis by subcellular and density gradient fractionation, and polyacrylamide gel electrophoresis techniques showed that livers with high activity exhibit a dual enzyme distribution in lysosomes (app. 76%) and endoplasmic reticulum (named "microsomal enzyme form"; app. 24%), whereas those with low activity not only lack the microsomal enzyme form - presumably due to the absence of egasyn - but also display reduced lysosomal enzyme activity.