Data are presented concerning the distribution of the ABO, MN and Rh blood groups and the saliva AB-secretion in the endogameous population of Ivád (Ivády family). Although a greater frequency of the A-, M-, and D-genes was observable, the differences between the Ivády family and the surrounding populations from where the family originated, were not significant. Isoagglutinin and bacterial agglutinin titres showed a narrower variation at Ivád than in Budapest. No agammaglobulinaemia or a higher frequency of Rhesus isoimmunization was observable. The most prominent feature of the Ivády isolate was the conservation of the original gene frequencies. The effect of random genetic drift or of selection was not demonstrable as far as the genes investigated are concerned.