Hemoglobin Hasharon (alpha(2) (47 his)(CD5)beta(2)) was found to comprise only 16-19% of hemolysates of carriers. These heterozygotes appeared to have mild, compensated, hemolytic anemia. Hb Hasharon was more heat-labile than hemoglobins A, S, or C. Its specific activity was higher than that of Hb A after administration of (59)Fe to two carriers. When hemoglobin synthesis by bone marrow cells was studied in vitro, about 18% of incorporated leucine appeared in the Hb Hasharon fraction. It is suggested that Hb Hasharon is unstable in vivo, and that mild hemolytic anemia and a relatively small decrease in its concentration in hemolysates result from its denaturation within red cells. Decreased synthesis, which appears to be the major cause of the small amount of abnormal hemoglobin, may protect heterozygotes from clinically significant hemolytic anemia.