Diamond, Rust & Westphal (1969) originally showed that pregnant guinea-pig plasma binds progesterone about 100 times more firmly than cortisol. Work along this line yielded a competitive protein-binding method for the assay of progesterone in plasma during pregnancy. Plasma was obtained from pregnant Casia guinea-pigs in the latter part of gesta-tion. [1-2-3H]Progesterone (6 ng, 33 Ci/mmol) in ethanol was evaporated to dryness and dissolved in 30 ml 0·04 m-phosphate buffer, pH 7·4. Fifteen microlitres guineapig plasma were added and mixed gently for 1 h at room temperature. To 100 μl plasma, 200 μl methanol were added and the mixture kept at − 20 °C for 10 min. After centrifugation an appropriate volume of the supernatant was transferred to a small glass test tube and evaporated to dryness. Benzene (100 μl) and heptafluorobutyric anhydride (3 μl) were added to the residue and the sample was kept at 62 °C for 30