The intratumoral activity of beta-Glucuronidase (total and free) was determined in surgical specimens of malignant tumors of the lung (N equal to 20) and of the breast (N equal to 15) from patients. A free beta-Glucuronidase-activity from about 90 mU per g tissue was found but there was no difference between both tumor types, and the beta-Glucuronidase-activity, as it seems, is not dependent on histological characteristics of the tumors. The findings are discussed under the view of the key function of the lysosomal enzymes in the concept of the "Cancer-Multistep-Therapy" by M. von Ardenne.