The enzymatic pattern of five enzymes involved in the purine salvage pathway, namely purine nucleoside phosphorylase (EC 2.4.2.1), adenosine deaminase (EC 3.5.4.4), 5'-nucleotidase (EC 3.1.3.5), alkaline phosphatase (EC 3.1.3.1), and hypoxanthine-guanine phosphoribosyltransferase (EC 2.4.2.8) has been evaluated both in human intestinal and breast carcinomas and compared to that of normal tissues. A higher level of hypoxanthine-guanine phosphoribosyltransferase was associated with tumor tissues. This metabolic alteration should lead to an elevated synthesis of nucleotides in cancer cells, might confer selective growth advantages to neoplastic tissues, and account, at least in part, for the difficulties encountered in the chemotherapy of human tumors, by using compounds affecting only the purine de novo biosynthesis.