The direct assay of serum progesterone after denaturation of the binding proteins was investigated. 50ul of patients' serum was diluted with 750ul phosphate buffer (0.05M, pH 7.4) and heated to 65 degrees C for 20 minutes. After cooling, 300ul of the treated serum was reacted with a rabbit antiserum to progesterone-11 alpha-hemicuccinyl-bovine serum albumin conjugate (Bioclin, U.K) and 1,2,6,7, tritium labelled progesterone. Separation of bound and free fractions was achieved with dextran coated charcoal. The method correlated well (r = 0.98) with an established method involving ether extraction of progesterone prior to assay. The mean sensitivity was 2.01 nmol/L (range 1.90-2.23nmol/L). The proposed method considerably shortens assay time and removes a tedious and imprecise stage in the conventional method involving extraction of serum.