111In-antimyosin F ab (AM) myocardial scintigraphy was carried out in (A) 13 patients with acute myocardial infarction (9.9 +/- 2.2 days from the onset) and (B) 9 with myocarditis and/or dilated cardiomyopathy. Forty eight hours after injection of AM, the patients were injected with 74 MBq (2 mCi) of thallium-201 (TL). The two sets of Planar and SPECT image were obtained simultaneously using dual energy window sets. In group A, positive focal AM uptake was demonstrated in 12 (92%) patients. Higher AM uptake was observed in patients who had PTCR/PTCA. By combination with TL, it is useful to detect inferior infarction and to differentiate old from acute infarction. Dual SPECT images gave precise information about the infarcted area. In group B, positive diffuse AM uptake was demonstrated in 7 (77%) patients. In conclusion, AM myocardial scintigraphy was proven to be useful for the assessment of acute necrosis after myocardial infarction but also on-going necrosis of myocarditis and/or myopathy.