Over half the patients with proven ectopic pregnancies have negative urinary slide or tube pregnancy tests at the time they present with symptoms. Because of the morbidity and mortality associated with unrecognized ectopic pregnancy, the need for an accurate, rapid, sensitive, and specific method for measuring low levels of human chorionic gonadotropin (hCG) is apparent. A three-hour radioimmunoassay (RIA) for serum hCG with a sensitivity of 5 mlU/ml was developed which utilizes an antiserum generated against the beta suhunit of hCG,125I-hCG for tracer purposes, a highly purified hCG standard, and dioxane for separation of hound from free hormone. Blood samples were obtained from 52 patients with suspected ectopic pregnancies. Twentyfour of these patients had a surgically proven ectopic gestation. The blood samples were analyzed in the above described RIA, and in the standard (24-hour) double antibody beta-hCG RIA. There were no false positive or false negative results by either assay. These results indicate that the dioxane modification of the beta-hCG RIA is an accurate and rapid method for confirming the diagnosis of ectopic pregnancy.