ABSTRACT Saturation assays for progesterone and 17-hydroxyprogesterone have been discussed. These methods have a sensitivity of 0.06 ng/ml and satisfactory precision and reproducibility for physiologic concentrations. Practical points of technique and a system for analysis of data are presented. Steroids that can be converted to progesterone or 17-hydroxyprogesterone, either chemically or enzymatically, may also be measured by these techniques.