On the basis of electron microscopic evidence, a preparation of microvilli obtained from the surfaces of placental villi was previously considered by us to represent syncytiotrophoplast plasma membrane. The present investigation was undertaken to obtain independent biochemical evidence on the nature of the preparation. Protein, lipid, carbohydrate, sialic acid, phospholipid and cholesterol concentration were determined and found to be consistent with analyses of membranes obtained from other sources. Enzyme membrane markers (5'-nucleotidase, (Na+ + K+)-ATPase and alkaline phosphatase) were also assayed, and found to be considerably enriched in the microvillous preparation compared to the whole placental homogenate. These findings, together with other evidence, support the contention that the preparation is indeed syncytiotrophoblast plasma membrane.