Human amniotic epithelium from patients with normal pregnancy (normal amniotic epithelium) and from patients with materno-fetal rhesus incompatibility has been examined under the light and electron microscope. In comparison with normal amniotic epithelium, the epithelium in rhesus incompatibility displays the following morphological changes: 1. Unequal distribution and variable shape of the plump microvilli 2. Considerable narrowing of the intercellular spaces 3. Vacuolization of the cytoplasm 4. Absence of glycogen granules 5. Thickening of the basal membrane. There is a direct correlation between the morphological changes in the amniotic epithelium and the severity of the disease. Since rhesus factors are present only in erythrocytes, it is probably that these changes are not based on a direct immune conflict. On the contrary, they are more likely due to fetal hypoxia and to the cytotoxic effect of unbound, unconjugated bilirubin in the amniotic fluid.