Electron micrographs of erythroblastic inclusions from sections of marrow immunogold-labeled with mouse monoclonal antibody. Inclusions from a case of β-thalassemia major show a positive reaction with antibody against human α-globin chains (A, B) and no reaction with antibody against human β-globin chains (C). B shows part of the inclusion in A at higher magnification. There is virtually no labeling of inclusions in a control section that was reacted with normal mouse serum instead of a monoclonal antibody (D). Magnifications: A, × 20,000; B, × 59,000; C, × 23,000; D, × 35,000.