In this monograph, the chemopreventive effects of enterally administered proteases (trypsin, chymotrypsin, and papain) have been documented in a series of animal experimental tumor models. The experimental evidence demonstrates a significant inhibition of growth of both the primary tumor and the metastatic disseminations. Survival in animals treated with proteases is significantly longer than in untreated animals. The results confirm the fundamental correlation between early initiation of therapy and consequent growth of the tumorous disease. Comparable results have been shown in solid tumors in animal models (melanoma and Lewis lung carcinoma) and in human tumors (pancreatic and breast cancers). In this article, details of the known mechanisms of systemic actions of enterally administered proteases are documented and their relationship with cancerogenesis is discussed.