This is a selective survey of recent publications dealing with theoretical and established applications of steroid hormone radioimmunoassay procedures applied to clinical obstetric investigation. This subject is reviewed in three sections: first, basic principles of steroid hormone radioimmunoassay methodology; second, steroidogenesis in normal pregnancy and its relation to patterns of steroid hormones measured in maternal and fetal circulations; third, applied steroid radioimmunoassay technology as it is being used in established and potential clinical applications. It is concluded that steroid hormone radioimmunoassay procedures have been and will continue to be a highly productive technology applied to clinical obstetric investigation.