The development of knowledge concerning the phenomenon of transplacental hemorrhage into the maternal circulation is reviewed, and evidence is presented that the acid elution technic provides a simple and accurate method of diagnosing this condition. The data in the present study suggest that transplacental hemorrhage is not a random phenomenon but is related to obstetric abnormalities and procedures, and the relevance of this finding to the management of the Rh negative unsensitized mother is discussed. It is suggested that the acid elution technic will find useful applications in the diagnosis of neonatal anemia due to massive transplacental hemorrhage, and in the further elucidation of the mechanism of Rh sensitization.