The importance of Ludwik Hirszfeld's contribu tions to our knowledge of the ABO blood group system has been affirmed in various ways: by Karl Landsteiner (1931) in his Nobel Address, by Hirsz feld's election to the Presidency of the Blood Group Section of the Second International Congress of Blood Transfusion (Paris, 1937), and by the naming in his memory of the Hirszfeld Microbiological Institute in Wroclaw (Milgrom, 1954). Despite this, however, one very important part of his work on the blood groups still awaits appreciation and even recognition?namely, his early, explicit awareness of the relevance of the ABO genes to natural selection. Accordingly, an attempt is made in this communication to draw together his various com ments on the subject, under headings of Climate, Age and Sex, Social Class, Disease, and Race.