Summary and Conclusions: Specimens of blood of 131 families with 642 children were examined for the agglutinogens A, B, M, and N. The agglutinogens M and N seem to be inherited entirely independently of the agglutinogens A and B, confirming the observation of Landsteiner and Levine. Concerning the heredity of the agglutinogens M and N, two exceptions to the theory of a single pair of allelomorphic genes were found, but in neither case could the possibility of illegitimacy be excluded. Concerning the heredity of the agglutinogens A and B, one exception to the Bernstein theory was found, but here again the possibility of illegitimacy could not be excluded. Considering the size of the series of cases studied (642 children), a certain percentage of illegitimacies is rather to be expected. Since the agglutinogens A, B, M, and N together can detect one third of all illegitimacies, the three exceptions found would imply the presence of only nine illegitimacies. We therefore believe that our results speak well for the theories of heredity discussed.