To explore the role of calcium in the materno-foetal relationship we decided to study the surface ultrastructure of the human placenta. Fresh pieces of tissues were obtained from central and peripheral parts of the maternal surface of human full-term placentas, processed and then examined with the Scanning Electron Microscope. Calcium depositions could only be seen at higher magnifications in forms of flecks, plaques, and concretions. They were frequently found in firm association with the tips of microvilli projecting from the apical parts of the syncytiotrophoblasts, which led to the clumping of those tips. Regional variations in the distribution of calcium deposits were apparent. Our findings indicate that placental calcification is a continual process occurring simultaneously in various parts of the placenta to varying degrees. Moreover, it seems possible that the process of placental calcification is of clinical and pathological significance bearing relationship to both maternal and foetal conditions.