A double-antibody radioimmunoassay for PMSG, especially for meauring PMSG in cattle blood after exogenous application, has been developed. A rabbit antiserum against PMSG and pure PMSG for radioiodination were used. There was a strong cross-reaction against equine LH and FSH, but the slight cross-reaction against bovine LH and FSH could be eliminated by adding bovine LH to each tube during the assay. Unspecific, interfering influences of equine or cow serum could be eliminated by adding a constant amount of PMSG-free serum to each tube. PMSG added to 200 microliter of serum could be recovered by this method with a mean of 90 . 5 +/- 9 . 9%. Inhibition curves obtained with pregnant mare serum or cow serum after administration of PMSG were parallel to those obtained with the PMSG standard preparation. The intra-assay coefficient of variation (CV) was 6 . 9%. The inter-assay CV was 12 . 6%. Sensitivity of the assay was 1 mi.u. PMSG/tube. Values of PMSG measured in the serum of pregnant mares by this assay were comparable with those obtained by a bioassay on the same samples. PMSG was still measurable in blood serum about 10 days after injection of 1500-3000 i.u. PMSG. After infusion of 12,000 i.u. PMSG for 3 h (2 heifers), the half-life of PMSG was found to have two components, one of 51 or 40 h and a slower one of 123 or 118 h.