1. Small differences in the antigenic structures of human normal adult hemoglobin and sickle cell anemia hemoglobin were found to exist. A difference in the serologic behavior of these two hemoglobins attributable to a factor other than that of antigenic specificity was also demonstrated. This factor is thought to pertain to differences in the physicochemical properties of the 2 hemoglobins.
2. Rabbit antiserums failed to show a difference in antigenic specificity, but with chicken antiserums a difference was clearly demonstrated when the in vitro reactions were performed under conditions which produced maximal precipitation. Results suggest that these 2 hemoglobins have a predominance of common antigenic determinants but a small number that are unique for each of the types.
3. Relatively large differences in specificity were found to exist between fetal hemoglobin and adult hemoglobin which suggest that only a few antigenic groups are shared in common by the 2 types.
4. The second, minor hemoglobin in patients with sickle cell anemia was shown to have an antigenic specificity that was either identical with or very similar to that of fetal hemoglobin.