A newly developed HPLC procedure (1) was used to study the percentages ofGγandAγchains in the Hb F from newborn and patients with sickle cell anemia. The method gives data comparable to those obtained with a more laborious chemical procedure. However, the presence of theAγTchain, i.e., theAγchain with an Ile→Thr substitution at position 75, interferes with the determination of theGγtoAγratio because theaγtandGγchains have the same chromatographic mobility.