Abstract A marked selection for allotype b4 over b9 was observed for anti-streptococcal antibodies produced in rabbits heterozygous for allotypes of groups a and b. Less than 2% of the streptococcal antibodies was of allotype b9 while 20% of the IgG in preimmune sera from the same rabbits was of allotype b9. No such effect was observed with respect to the group a allotypes. In spite of this marked selectivity in b4,b9 heterozygotes for the production of antistreptococcal antibodies of allotype b4, no evidence for linkage between the gene controlling this allotype and immune responsiveness to streptococcal antigens could be detected. Equal amounts of antibody were produced by all immunized offspring (b4,b4; b4,b9; or b9,b9) of heterozygous rabbits.