HUMAN placental alkaline phosphatase has several unique properties. It is stable to heating at 65 °C (ref. 1), is immunochemically distinct from the isoenzymes in other adult organs2,3, and has a large number of allelic variants4–6, with a ‘degree of heterozygosity’ much larger than that of any other human enzyme studied to date7. If placental phosphatase is specific to fetal development, it would be logical to expect that a similar isoenzyme would be present during fetal development in other mammalian species. This is not the case, however, as several species, including rhesus monkey, African green monkey and baboon, do not have in their placentae an isoenzyme which resembles human placental phosphatase8. The presence of the placental isoenzyme in humans therefore seems to be a late evolutionary event. To determine the species specificity of this enzyme, we have used the criteria which distinguish the human placental isoenzyme, including inhibition by 5 mM L-phenylalanine1 and insensitivity to inhibition by 0.1 mM bromotetramisole9, to compare the enzymes in species closely related to man. We report here that of the species studied, only the chimpanzee and orangutan have an isoenzyme resembling that in humans.