Studies of anaemia occurring during pregnancy have been of two main types. The first type, from which information of great value has been collected, is the mass survey of large series of cases. Studies of such a nature have the disadvantage that the less common varieties of pregnancy anaemia are pooled amongst the more common, a feature which is largely responsible for the perpetuation of traditional rather than factual ideas on the subject of these less common forms. The second type of study, of which the present is an example, consists of a more intensive and detailed investigation of individual cases with the main purpose of interpreting pathogenesis in terms of deviation from that which is physiological. Anaemia and pregnancy are common associations. It is important to distinguish between anaemia which is apparently due to pregnancy and anaemia which is associated coincidentally with pregnancy. In the present enquiry, the phrase "anaemia during pregnancy" generically denotes that a case is pregnant and anaemic at the same time. " Anaemia of pregnancy" and "pregnancy anaemia" are used synonymously to indicate that the anaemia is conditioned directly by pregnancy; it is perhaps too strong to refer to such anaemia as being due to pregnancy. ". Anaemia complicated by pregnancy" indicates that pregnancy occurs coincidentally with some disorder of the blood which ordinarily occurs quite apart from and is quite unrelated to pregnancy. The present investigation has sought to establish normal standards for the less commonly noted physical features of red cell fragility and red cell size during normal pregnancy, and to investigate these features in mild and severe grades of anaemia during pregnancy, followed through to the puerperium. At the same time, full haematological investigations were carried out and the results of various types of treatment followed. The type of response to treatment was used as an important aid to the diagnosis of the type of anaemia. That the investigation of the red cell fragility in pregnancy might afford information of value was suggested by Dr. J.M. Vaughan on the evidence of a single case under 2 her own observation some time previously in which the fragility was increased. The observations of Cassells (1938) also suggested that with an accurate technique for fragility estimation the red cells during pregnancy could be shown to behave differently from other forms of anaemia as regards their fragility. As no controlled work had been published on the subject, it was necessary to establish standards of red cell fragility for pregnancy uncomplicated by anaemia, and to compare these standards with the findings in pregnancy complicated by anaemia. On the assumption that red cell fragility is at least in part related to red cell thickness (Haden, 1934, Dacie and Vaughan, 1938), estimations of mean cell volume and mean cell diameter were an essential part of the investigation. Red cell counts, haemoglobin estimations, reticulocyte counts, white cell counts, and estimation of plasma bilirubin were part of the routine investigation.