The recent development of a clearer understanding of pernicious anemia and of the conditions which are allied to it has made more prominent the anemic states which do not respond to adequate therapy with liver extract or with iron. These refractory anemias have been long recognized but have been classified loosely as severe secondary, aregenerative or aplastic anemia. Insufficient information has been available, however, to warrant any conclusions concerning the fundamental nature of the conditions. For this reason, a clinical and experimental study of refractory anemia has been made at the Hospital of the Rockefeller Institute. In the series, all those patients have been included who were referred to the institution with anemia of severe degree which did not respond to the usual therapeutic methods. In this way a suitable cross section of the problem as it presents itself to the practitioner was obtained. The diagnosis of refractory anemia can