SUMMARY Antibodies to norethisterone and norgestrel were raised in rabbits by injecting the steroid 3-carboxymethyloxime coupled to bovine serum albumin. Within 6 months of the first injection, antisera were obtained which gave 50% binding of the labelled steroid at dilutions of 1:100000; after treatment with Rivanol the antisera had titres of 1:20000 to 1:30000. Labelled ligands were prepared by iodination of the tyrosine methyl esters of the steroid oximes; minimum specific activities of 100 to 150 Ci/mmol were obtained. Dextran-charcoal suspension was used for the separation of bound and free labelled steroid. The effect of changes in the incubation conditions on the sensitivity of the assay were investigated. The reliability criteria for the assays were satisfactory, blank values were low and the mean recoveries were greater than 90%. The sensitivity of the method corresponded to a concentration of the steroids in plasma of 30 pg/ml. The coefficient of variation for replicate analyses on the same sample varied from 13·5 to 16%. Apart from the parent steroid, the 3-oxime and the tyrosine methyl ester of the oxime, significant cross-reaction with the antiserum was also obtained with the dihydro and tetrahydro metabolites of the progestogens. After administration orally of 1 mg norgestrel or 1 mg norethisterone to human subjects, peak levels of the steroids in plasma were obtained within 1 to 2 h. Norgestrel was metabolized more slowly than norethisterone. For the periods 2–8 h and 8–24 h after administration of norethisterone the half-lives were approximately 3 h and 5 h respectively; for norgestrel the half-lives were 2·8 h and 10 h. Twenty-four hours after administration, the mean levels of norgestrel and norethisterone in plasma were 1·5 and 0·2 ng/ml respectively corresponding approximately to 0·4% and 0·05% of the dose in the total plasma volume.