Myocardial perfusion scintigraphy was performed in 30 intact anesthetized dogs following intracoronary injection of99mTc-labeled human albumin microspheres (HAM). The normal dog myocardial image pattern was established. This technic detected myocardial infarcts resulting from small vessel occlusions and flush coronary branch occlusions when the coronary angiogram appeared normal. Myocardial infarcts as small as 1½ cm were seen. Toxicologic studies in 47 dogs failed to demonstrate significant changes in blood pressure, heart rate, electrocardiogram, or serum CPK activity following intracoronary injections of HAM in doses up to 200 times that required for adequate myocardial scintigraphy. Gross and microscopic examination of hearts 1-6 weeks after HAM injection showed minor abnormalities also seen in matched controls receiving coronary arteriography alone. Preliminary results in 33 patients substantiate the safety of this technic. Human myocardial scintigrams reported here reveal anatomic detail not previously demonstrated by other external imaging methods.