In a previous communication in this journal one of us (THJH) discussed evidence indicating that the simultaneous presence of anα-chain deficiency (α-thalassemia-2) and a heterozygosity for theβchain variant Hb S or Hb C results in a decreased production of the abnormal hemoglobin (1). It was shown that the level of Hb S in Hb S heterozygotes is decreased to less than 30% when anα-thalassemia-2 homozygosity (indicated by the -α/-α;βA/βSgenic arrangement) is also present, that this level varies between 30 and 40% if anα-thalassemia-2 heterozygosity (the -α/αα;βA/βSgenic arrangement) is present, and that the level is more than 40% but less than 50% if fourαchain structural genes are active. Data from several studies (reviewed in 1, 2, and 3) have suggested some 2 to 5% Hb Bart's (orγ4) is present in the blood of Black newborn with a homozygosity forα-thalassemia-2 (irrespective of aβchain variant being present), less than 2% Hb Bart's in Black newborn with anα-thalassemia-2 heterozygosity, and no detectable amount of Hb Bart's in newborn with four activeαchain genes.