Serum samples (93 total) from 62 different patients were analyzed by quantitative radioimmunoassay (RIA) and by the Biocept-G radioreceptorassay (RRA) for detection of human chorionic gonadotropin (hCG). The ratio of sample counts to reference control counts (S/R) was determined for each RRA. For the 44 serum samples having S/R ratios between 0.295 and 1.20 (corresponding to hCG levels of 4600 and 89 mIU/ml, respectively), there was good correlation between the S/R ratio by RRA and the serum level of hCG by RIA (correlation coefficient = '0.92). When serum samples were analyzed in as many as 4 different Biocept-G kits, the same S/R ratios were obtained. The S/R ratio reliably predicted the trend in serum hCG titer in 7 patients during treatment for gestational trophoblastic disease. The usefulness of the S/R ratio in obtaining semiquantitative results with the Biocept-G RRA is discussed.