Work from Strang and other laboratories has established that the 2-/16 alpha-hydroxyestrone ratio is inversely correlated with the risk for breast and cervical cancer. In order to measure these metabolites in urine samples, it is essential to have an assay for these compounds that is both sensitive and reproducible. The present paper describes such an ELISA assay, which overcomes problems that existed in prior approaches to measuring these compounds. The new ELISA procedure supplies greater sensitivity and reproducibility than earlier assay procedures. The ELISA assay has also been found to correlate well with the GC-MS procedure of Adlercreutz.