Monoclonal antibodies were raised against alkaline phosphatase (ALP) from the malignant human cell line Hep2, a derivative of HeLa, which was established from a cervical carcinoma. Two forms of ALP are found in Hep2 cells resembling, but not identical to, the normal placental and intestinal ALPs. Seven monoclonal antibodies were raised against Hep2 ALP. All of these reacted nearly identically with normal placental ALP and the placental-like ALP from Hep2 cells. Three of these antibodies exhibited reduced reactivities for either the common type 2 or 3 allelic variants of placental ALP. From these binding studies, the placental-like ALP from Hep2 cells appears to be a slightly modified form of the normal type 1 placental ALP. One of the seven antibodies bound to the adult and fetal intestinal ALPs, but more strongly to the adult form, suggesting that the Hep2 intestinal-like ALP is a form of the normal adult intestinal ALP.