To understand the differences between the rat intestinal alkaline phospha-tase isozymes rIAP-I and rIAP-II, we constructed structural models based on the previously determined crystal structure for human placental alkaline phosphatase (hPLAP). Our models of rIAP-I and rIAP-II displayed a typ-ical a⁄btopology, but the crown domain of rIAP-I contained an additional b-sheet, while the embracing arm region of rIAP-II lacked the a-helix, when each model was compared to hPLAP.