Using both direct and competitive binding studies it is demonstrated that antibodies to beef myoglobin raised in sheep are able to distinguish between beef and sheep myoglobins although these two proteins differ by only six of the 153 amino acid residues. By contrast, antibodies to beef myoglobin raised in rabbits, dogs and chickens bind almost equally well to beef and sheep myoglobins. It is also shown that antibodies to beef myoglobin raised in sheep have a lower avidity for beef myoglobin than do antibodies raised in more distantly related species. Furthermore, only 50% of the specific anti-beef myoglobin antibodies isolated from sheep antisera will bind to sheep myoglobins whereas 100% of the specific antibodies isolated from the antisera of the other immunised species will bind to sheep myoglobin. It is suggested that antibodies to beef myoglobin are raised to those surface regions which are topographically altered as a result of sequence differences from the host's own myoglobin. When the host animal is evolutionarily distant these sequence differences are considerable and antibodies are raised to the entire surface of the molecule. However, when the host's myoglobin is very similar in sequence to beef myoglobin (as is the case when using sheep as the host animal) antibodies are made only to surface regions affected by the sequence differences. Some of these antibodies--those to the regions of greatest difference--will bind weakly if at all to sheep myoglobin, while those directed to areas of lesser difference will bind well to sheep myoglobin.