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Assays of urinary human chorionic gonadotropin (HCG) in normal and abnormal pregnancies performed by a hemagglutination inhibition method are presented. Antisera were produced in rabbits. An HCG preparation of pooled urine of pregnant women was used as a laboratory standard. 1st morning urine samples were obtained from a large group of patients mostly in aid of diagnosis of pregnancy. Patients with a variety of pathologic conditions also supplied specimens. There were a total of 1021 assays on 691 patients. It was possible to follow only 492 patients for 588 assays. There were 10 false positive and 2 false negative results for an overall accuracy of 98.0%. The mean level of HCG in normal pregnancies peaked at about 70,000 IU in the first trimester then fell to a constant level of 20,000 IU during the 2nd half of the pregnancy. The 95% confidence limits of the levels in normal pregnancy were from 10,000 to 350,000 IUD in the latter part of the first trimester and 2500-150,000 in the remainder. The levels of HCG in patients with clinical evidence of threatened abortion were determined. There were 86 such pregnancies, 47 of which had spontaneous abortion, while the remaining 39 went to term. The mean levels of the urinary HCG in the patients that aborted were consistently lower than those with normal pregnancies. Only during the period of 57-110 days after the last menstrual period were the values significant (P less than .001). 4 out of 6 patients with hydatidiform moles had titers of HCG greater than the normal range. HCG tests are also useful in following patients treated with chemotherapy for choriocarcinoma. The immunologic assay of HCG is a rapid, uncomplicated method of determining the level of HCG. It is of value in aiding the clinician in diagnosis, prognosis, and management of certain pathologic conditions of pregnancy.