Polyacrylamide gel electrophoresis (PAGE) in the presence of urea, acid, and Triton X-100 was used for determination of theGγtoAγratio in human Hb F. The data compared most favourably with results obtained by a HPLC procedure and by a chemical procedure. Moreover, its accuracy and reproducibility was determined. The presence of Hb A2in a sample with Hb F level below 10% interferes with the determination because of the nearly identical electro-phoretic mobilities of theδandGγchains. Thus, the removal of Hb A2is required. Samples free of Hb A2but with a Hb F level as low as 2% can be analyzed with an accuracy of about 5%. The PAGE method has been applied to the Hb F of 63 subjects withβthalassemia and related conditions, and the results of theseanalysesare included in this communication.