Normal, term, human placentas are a rich source of a 15-hydroxyprostaglandin dehydrogenase. The enzyme is extremely labile, and partial purification could be achieved only after stabilization with glycerol. The instability of the enzyme and its K(m) for NAD are indications that it is different from the 15-hydroxyprostaglandin dehydrogenase isolated from swine lung. Human placental tissue should provide a very useful source from which large amounts of highly purified 15-hydroxyprostaglandin dehydrogenase may be obtained.