In rats with adjuvant-induced arthritis, the effect of (+)-catechin (CA) and 0-(beta-hydroxyethyl) rutosides (HR) on the activity of certain lysosomal acid hydrolases, viz., beta-glucuronidase, beta-N-acetyl glucosaminidase and cathepsin D in serum, liver, kidney and spleen and the stability of liver lysosomes was studied. The activity of these enzymes in arthritic tissues and serum increased significantly. The total activity of beta-glucuronidase in the lysosome-rich fraction from arthritic liver was appreciably decreased, while its release was significantly increased. These results demonstrate the fragility of lysosomes in arthritic tissues. Administration of CA or HR to the arthritic animals was found to have a prophylactic action by stabilizing liver lysosomes and reducing the free lysosomal enzyme activities in serum, liver, kidney and spleen. CA was more effective than HR.