The radioreceptorassay developed to determine serum luteinizing hormone level in the cynomolgus monkey was evaluated for its usefulness in early pregnancy diagnosis by the detection of serum chorionic gonadotropin (CG). Blood samples were collected at weekly intervals from the 1st to the 5th week after conception to determine changes in circulating levels of CG. In the pregnancy cases, serum CG levels increased to above 50 μg/ml in almost all animals. By the determination of CG 3 weeks after conception, 86% of all pregnant cases exhibited a positive response. Cases that were negative 3 weeks after conception were followed by a repeated test in the next week. According to this test schedule, 95% of the pregnant cases were detected by 4 weeks after conception, and 5% were undetected as negative responses because their CG levels were low.