Significant alkaline phosphatase activity has been found in human placenta and fetal brain tissues. The enzyme is present in cytosol as well as in microsomal fractions with a relatively higher activity in the latter fraction. The activity of the enzyme in both the tissues shows a progressive increment with gestational age of the tissues; however, the term placental tissues does not register any further increase in the enzyme activity. In contrast to fetal brain, the placental enzyme demonstrates varying degrees of heat-stability depending on the gestational age of the tissue with preponderance after 14 weeks. Both fetal brain and placental enzymes demonstrate a characteristic activation after pre-exposure at pH 5.0; however, after 14 weeks the placental enzyme cannot demonstrate such acid-pH-induced activation. Dialysed enzyme preparation of both the tissues can demonstrate acid-pH-induced activation phenomena only when the pre-exposure is carried out in the presence of inorganic orthophosphate where arsenate acts as an inhibitor.