A 5-yr-old girl with hemoglobin E-β thalassemia was discovered in a family of mixed origin. The father is Iranian (β-thalassemia trait) and the mother is Burmese (hemoglobin-E trait). Hemoglobin synthesis was studied in vitro in the blood of the proposita and family members. In the subjects with hemoglobin E trait the ratio of the quantity of hemoglobin A to hemoglobin E was 3:1. However. the βA/βE synthesis ratio in reticulocytes was in the range of 1.5-2.18, and the specific activity of βE was 31%-49% greater than βA, suggesting instability of hemoglobin E with preferential destruction of abnormal hemoglobin. The blood of the proposita exhibited only hemoglobin F and hemoglobin E and reticulocytes and bone marrow showed no βA synthesis. This Iranian β-thalassemia gene is therefore of the β° type. The βE/α synthesis ratio (approximately 0.74) in blood of the proposita was similar to the βA/α ratio in mildly affected relatives with thalassemia trait. These results suggest that the severity of the hemoglobin E-β thalassemia syndrome is attributable to both instability and defective synthesis of hemoglobin E in association with absent βA synthesis due to a β° thalassemia gene.