EM evidence indicated that a preparation of microvilli obtained from the surfaces of human placental villi represented syncytiotrophoplast plasma membrane. Independent biochemical evidence on the nature of the preparation was obtained. Protein, lipid, carbohydrate, sialic acid, phospholipid and cholesterol concentration were determined and were consistent with analyses of membranes obtained from other sources. Enzyme membrane markers (5''-nucleotidase, (Na+ + K+)-ATPase and alkaline phosphatase) were assayed, and were considerably enriched in the microvillous preparation compared to the whole placental homogenate. The preparation apparently is syncytiotrophoblast plasma membrane.