A 23-yr-old man of Greek-Italian ancestry with mild anemia was found to be heterozygous for HbD (Punjab) beta121 glu leads to gin and beta-thalassemia. HbA was not detected upon electrophoresis of the subject's hemolysate, and no synthesis of betaA globin was demonstrated after incubation of peripheral blood or bone marrow with 3H-leucine. The thalassemia gene was thus of the betao variety. The betaD/alpha synthesis ratios were almost equally unbalanced in the blood and bone marrow: 0.53 and 0.61, respectively. The mother of the propositus had beta-thalassemia trait. In peripheral blood the betaA/alpha synthesis ratio was 0.38. The mutant betaD gene thus appeared potentially capable of directing the synthesis of globin chains as efficiently as a normal betaA gene. The mildness of the HbD-betao-thalassemia syndrome appeared to be due to the maintenance of a relatively high total beta/alpha synthesis ratio in the presence of a physiologically neutral structural mutation.