Summary: Thirty-nine per cent of the normal serums from 422 rabbits contained group-specific agglutinins for human A cells, 16 per cent specific agglutinins for B cells and 38 per cent failed to agglutinate any type of human cell. There is considerable evidence to show that the presence or absence of specific A agglutinins in the serum of rabbits is typically Mendelian in its inheritance.