By means of fractional denaturation, it is possible to follow the progress of alkaline degradation of resistant hemoglobins. A characteristic pattern was established for the fetal compound. The alkali resistant hemoglobin fractions in Mediterranean anemia and in some acquired hematologic conditions (acute leukemia and chronic aregenerative anemia) behaved like the fetal pigment. In sickle cell anemia only 4 out of 11 specimens were found to have fetal hemoglobin, whereas the others seem to have a fetal-like compound. Similarly, in 2 members of one family with hereditary spherocytosis, the resistant pigment was also fetal-like. The significance of these findings for the hypothesis that the resistant hemoglobin fractions in these disorders represent either a continuation or a reactivation of the production of the embryonic pigment is discussed.