[en] A crude hCG with an activity of about 3,000 IU per mg was purified to 10,000-15,000 IU per mg of dry weight using Amberlite CG-50 chromatography combined with DEAE-Sephadex A-50 and Sephadex G-75. The alpha and beta subunits of hCG were prepared by urea-treatment of the hormone and were isolated by DEAE-Sephadex A-50 chromatography. Further purification of the subunits was achieved by gel filtration on a Sephadex G-57 column. For radioimmunoassay, hCG was iodinated by the DMSO-chloramine T method. Iodination of hCG with non-radioactive iodine revealed that the addition of DMSO to the iodination mixture seemed to reduce the iodination damage to the antigenic activity of the hormone. The hCG iodinated with non-radioactive iodine (accomplished by the DMSO-chloramine T method) was 1.5 times more immunoreactive in the hCG radioimmunoassay then the hCG iodinated by the usual chloramine T method. The radioimmunoassay of the hCG-beta subunit developed in our laboratory was satisfactory with respect to specificity. The hLH, hFSH, hTSH and hCG-alpha subunits tested cross-reacted very poorly in our assay system. Desialylated-hCG and subunits, in which the biologic potency was almost zero, also exhibited decreased immunoreactivity, about 30% of the native hormone with grossly unimpaired parallelism in their respective homologous radioimmunoassays. The concentrations of hCG and the subunits were determined on human sera from pregnant patients. The hCG levels reached to a peak at the first trimester of the pregnancy, but the hCG-beta subunits varied poorly in their concentrations throughout the periods of pregnancy. The hCG-alpha levels, on the other hand, showed two distinct peaks, in the early period and at the term of pregnancy. (J.P.N.)