Previous communications 1,2 concerning A2 hemoglobin in parents of subjects with thalassemia major reported normal levels in a small minority. Fetal hemoglobin percentage was determined in some of these individuals and was noted to be unexpectedly high in all but one. The single exception was the mother of 2 children originally diagnosed as having thalassemia major. Later investigation of this family resulted in reclassification of diagnosis to congenital nonspherocytic hemolytic anemia. It was suggested 2 "that patients diagnosed as having Mediterranean anemia do not represent a homogeneous group as regards pathogenesis and heredity." The present report details further observations concerning these families and presents evidence for heterogeneity of thalassemia major. Material and Methods The pedigrees of 3 subjects with thalassemia major who have been followed in the Pediatric Hematology Clinic of the Babies Hospital have been investigated for evidence of heredity transmission of the gene for thalassemia minor.