Arginase (EC 3.5.3.1) activity was determined in 54 colorectal tissues obtained from patients with primary colorectal adenocarcinoma as well as in serum of 45 patients and 65 healthy individuals. In patients, the preoperative values of the mean serum arginase activity and the activity in colorectal tumors were much higher than in serum of healthy subjects and control tissues. Two isoforms of arginase, anionic and cationic, were identified in colorectal tissues (normal and cancerogenous), and only one, the cationic form, in serum. These arginases were different from the main human liver cationic arginase (pI 9.3). The anionic colorectal arginase was identical with the human liver anionic isoform (pI 7.7), and the cationic arginase from colorectal tissues and blood serum with the human kidney cationic enzyme (pI 8.9). The total activity and the level of protein of the cationic arginase in colorectal cancer was higher than in control tissue, and it was also higher in serum of patients with colorectal cancer than in healthy subjects. Thus, it can be concluded that the increased arginase activity in blood serum and colorectal cancer in studied patients was due to the raised level of the cationic arginase and this isoform seems to be a discriminating parameter for assessing the presence of colorectal cancer.