Quantitative and qualitative data on alkaline phosphatase from 108 samples of amniotic fluid, and from maternal serum, cord serum, chorionic and amniotic membranes, placenta and meconium, are reported. a significant difference was observed between the amniotic enzymatic activity in early and late pregnancy. by exclusion fetal skin and urogenital tract are proposed to be the source of amniotic fluid alkaline phosphatase as the enzyme patterns obtained by starch-gel electrophoresis and thermal inactivation of amniotic fluid enzyme are different from the corresponding patterns obtained from other sources.