A radioimmunaassay method for the measurement of progesterone (P) in plasma after selective solvent extraction with hexane is described. Utilizing specific antisera against P with minimal or no cross reaction with other steroids, P was measured accurately in a volume of 0.5 ml of plasma. The range of P levels that could be measured per mi of plasma in the assay system described varied from 0.04 to 40 ng/ml. Comparison of the P levels obtained by this rapid assay and by o proven chromatographic system suggests adequate specificity of this quick method.