Purification, molecular weights, amino acid compositions, and partial sequencing of intestinal alkaline phosphatases (EC 3.1.3.1) from human adult, human fetal, and bovine sources is reported. Additional sequence information is presented for the bovine liver isozyme. Comparisons are made of the partial primary structures of intestinal alkaline phosphatases with those of the isozymes from liver and placenta. Homologies among these isozymes provide structural data to corroborate some concepts of the etiology of these isozymes and refute others.