Wistar-Furth (WF) strain rats have been shown to have a high incidence of spontaneous colon cancer. We have compared the alkaline phosphatase (AP) isoenzymes in colonic carcinomas of these inbred WF strain rats with the isoenzymes in the non-carcinomatous colon of the same strain and the normal colon of Wistar strain rats, from which the WF strain was derived. On electrophoresis of AP specimens partially purified by acetone fractionation, the non-carcinomatous colon was found to have three main isoenzymes, while the normal colon had two. Colonic carcinomas gave one band which migrated slower than any bands from either normal or non-carcinomatous colon. The electrophoretic mobility of AP from colonic carcinomas was retarded by neuraminidase treatment. This did not influence the isoenzymes from non-carcinomatous and normal colon. The inhibition patterns of the enzyme from colonic carcinoma by amino acids, inorganic phosphate and urea were different from those of the isoenzymes in the other two tissues. AP of colonic carcinoma was also the most heat-labile. There were no significant differences in the enzymic properties of the isoenzymes from non-carcinomatous and normal colon. The enzymic properties except for electrophoretic mobility were found to be the same between APs of colonic carcinoma and the placenta.