Specificity studies of the binding of R1881 to crude placental homogenate gave surprising results in that certain steroids increased the binding of [3H]R1881 rather than displacing it. While data for 'competing', i.e. displacing, steroids were similar to those reported by other authors, there have been no previous reports of increased binding due to added steroids. This increased binding was due mainly to an increase in capacity (about 10-fold). These data suggest that the placental steroid-binding protein is unusual in that there is a second steroid-binding site whose occupancy increases the stability of the protein, thereby increasing its capacity to bind R1881.