Several primate myoglobins cross-react with a rabbit antiserum to human myoglobin. 2. No immunoelectrophoretic differences are detectable among these reactions. 3. Differences can be detected in agar gel double diffusion reactions, cross-absorption, and quantitative precipitin reactions. 4. Myoglobins of man and the apes (Group I ) precipitate the maximum amount of protein from the anti-human myoglobin serum, give reactions of coalescence with each other in gels, and totally abolish reactivity of the antiserum with any primate myoglobin after absorption. 5. Myoglobins of baboons and macaques (Group II ) precipitate an average of approximately three-fourths as much protein from the antiserum as myoglobin of man and apes, and give reactions of coalescence with each other in gels but additional spurs are formed by Group I myoglobins. After absorption with myoglobins of baboons and macaques, antibody reactive with Group I myoglobins is left in the serum. 6. Myoglobin of the squirrel monkey (Group III ) precipitates approximately one-third as much protein from the antiserum as myoglobins from man and apes, and, when used for absorption of the rabbit antiserum, leaves behind antibody capable of reacting with Groups I and II myoglobins. 7. A hypothesis proposing similarity of structure of myoglobin molecules between members of each group, and differences of from one to three sites between members of different groups, is suggested.
