This monograph constitutes an attempt to determine some of the factors responsible for isosensitization to the Rh factor in pregnancy. It starts with a rather comprehensive review of the literature regarding the pathogenesis of erythroblastosis, both before and after the discovery of the Rh factor. This 26-page section is followed by a discussion of the author's own investigative attempts to determine whether or not the Rh antigen is present in cells other than erythrocytes. Tests for the Rh antigen by the absorption method were negative when placental tissue was used, confirming the previous reports in the literature that it is necessary for intact erythrocytes to enter the maternal circulation in order for isosensitization to occur.