A radioimmunoassay system for the glycoprotein β<sub>1</sub>SP<sub>1</sub>,, one of the pregnancy-associated proteins, has been set up. β<sub>1</sub>SP<sub>1</sub> levels in normal pregnancy have been established transversally. Clearance (t½) from the circulation postpartum ranged from 1.4 to 1.8 days. Several patients with trophoblastic disease, e.g., mola hydatidosa, choriocarcinoma and embryonic cell carcinoma, were followed longitudinally. Human chorionic gonadotropin (hCG), a highly reliable tumor marker in trophoblastic disease, has been determined concomitantly by radioimmunoassay. In all instances, hCG proved to be a better indicator of the presence of a tumor, because by the time β<sub>1</sub>SP<sub>1</sub>became undetectable, hCG could still be detected. Samples from a large multicenter study on trophoblastic disease were studied: out of 100 positive for hCG, β<sub>1</sub>SP<sub>1</sub> could be detected in 86; however, out of 21 samples in which hCG was not detectable, 1 was found to be positive for β<sub>1</sub>SP<sub>1</sub> . In a number of samples from patients with bronchial carcinoma, no β<sub>1</sub>SP<sub>1</sub>could be detected.