SummaryAn analysis of ABO blood‐type data on 3817 live births and their parents yielded the following main findings.1A deficiency of type‐A heterozygotes in the offspring. (This might be due to errors in typing.)2An excess of type‐B heterozygotes in compatible matings.3The incompatibility effect between type‐B offspring and type‐O mothers.I wish to express my deep gratitude to Profs. Y. Yerushalmy and C. Stern for their constant help and advice. Thanks are also due to Profs. S. Nissen‐Meyer, E. R. Dempster and D. Black‐well for their kind encouragement and to Miss L. Milkovitch, Miss s. C. Stevens and Mr W. Quinn for their assistance in obtaining the data and preparing th