Fibrinoid material from human placental villi was studied by electron microscopy. Four constituents were identified in the fibrinoid substance: a homogeneous, amorphous matrix; microfibrils; cellular debris and calcified corpuscles. Microfibrils, showing no periodicity, were very abundant and irregularly arranged. Cellular debris derive, probably, from degenerated syncytiotrophoblast, whereas the calcified corpuscles are similar to the Michaelis-Gutmann bodies. The present results suggest that fibrinoid material derives from both degeneration of the syncytiotrophoblast and synthesis by the residual Langhans cells and X-cells