New test systems using azocasein or double-labelled cytosol proteins from rat livers were developed to test the influence of a lot of possible effectors on lysosomal proteinases from rat liver. All as yet tested inhibitors of serine proteinases did not show substantial inhibition of the sum of all proteinases activity in the soluble part of rat liver lysosomes. Moreover at least 200 different substances were found to be without significant effects against these lysosomal proteinases. These proteinases are only effected by peptide aldehydes and other compounds which influence thiolproteinases. This provides further evidence that mainly the thiolproteinases (cathepsin L, H and B) are responsible for the proteolytic activity in rat liver lysosomes.