A high molecular weight form of human placental alkaline phosphatase has been detected in extracts of placenta at term by electrophoresis in starch gels containing 0.5 per cent Triton X-100. The enzyme has a mobility intermediate between the previously described A and B forms of the enzyme and has been called the 'M' form of placental alkaline phosphatase. The M form is the major form of the enzyme found in microvilli extracted from syncytiotrophoblast, though trace amounts of membrane-associated M form can be found in extracts of placentae which had previously been experimentally depleted of microvilli. The M form is present in both of the two recently described subfractions of placental microvilli (see Davies, Parry and Sutcliffe, 1981; Truman, Wakefield and Ford, 1981). A variety of experiments show that the M form is not an artefact of extraction. The characteristic mobility of the M form in starch/Triton gels is the same, whether the microvilli are extracted in butanol, chloroform/methanol, Nonidet P40, Triton X-100 or Na deoxycholate. Serological, heat-stability and genetic studies showed that the A and M forms contain the same enzymatic polypeptide. Gel filtration of butanol/H2O and butanol/saline extracts of microvilli provided an estimated molecular weight of the A form of 127000 and of the M form of 725000; these values were unaffected by the presence of Triton in the medium.