Thirty-one human placental membranes of various gestational ages (24 1/2 to 42 weeks) have been studied for their surface properties by use of a goniometer to observe drops of saline and olive oil applied at regular intervals to the amniotic and chorionic surfaces as they dried in air. Results show that the epithelial surfaces of the chorion and amnion and their interface are all appreciably hydrophobic. Surface energy decreased on the amniotic surface with fetal maturity while the surface energy of the chorion was significantly (137%) higher in those displaying premature rupture of the membranes than in others of comparable gestational age. This is explained on the basis that low-energy surfaces (for instance, Teflon) do not stick and provide boundary lubrication which facilitates movement. These properties are needed to avoid the buildup of local mechanical stress which could initiate rupture. It is speculated that the release agent-lubricant is surfactant derived from amniotic fluid and directly adsorbed onto the epithelial walls. On other tissues, the same surfactants have been shown to reduce surface energy, rendering them hydrophobic and imparting many desirable properties such as release, lubrication, and resistance to erosion.