A homologous enzymeimmunoassay (EIA) for plasma progesterone, using a horseradish peroxidase conjugate as enzyme label and an antiserum raised against a progesterone-11 kappa-hemisuccinyl/BSA conjugate, is described. The antiserum was covalently linked to microcrystalline cellulose to facilitate separation of bound and free steroid; this solid-phase antiserum was stable for at least nine months when stored at 4 degrees C. the freeze-dried enzyme label is also stable, having retained both enzymic and immunological activity for about four years. The EIA developed was specific and had the sensitivity (4.8 pg/tube) required for determining progesterone concentrations in in plasma samples collected at any time during the menstrual cycle. EIA of plasma samples provided results which were in good agreement with a well validated radioimmunoassay (RIA). The specificity and inter- and intra-assay coefficients of variation in the EIA were strictly comparable with those of the RIA. The method described has been in use for two years and has been assessed in external quality assurance programme established by the World Health organization and the United Kingdom Department of Health and Social Security.