Labelled monoclonal antimyosin antibodies have been proposed for the diagnostic imaging of acute myocardial infarction (AMI). In order to verify in the clinical practice the theoretical advantages of this new approach, we performed planar imaging with a commercial kit of 111In-antimyosin (111In-AM) in 17 patients admitted to our Coronary Care Unit with the diagnosis of AMI. The results were compared with the echocardiographic assessment of AMI and, in 9 subjects, also with 99mTc-pyrophosphate (99mTc-PYP) scintigraphy. Furthermore, the in-vivo kinetics of 111In-AM was investigated in 11 patients (blood pool activity curve; column gel-chromatography of the injected compound and patient serum). 111In-AM images showed a myocardial uptake in 16/17; 99mTc-PYP scintigraphy in 7/9. The site of AMI was correctly identified by 111In-AM in 14/17, was mistaken in one and impossible to evaluate in another (diffuse uptake pattern). AMI extent, qualitatively assessed in 111In-AM images was consonant with echocardiography in 8/17 and with 99mTc-PYP in 5 of 9 subjects studied also with this method. An apparent underestimation, in comparison with echocardiography was found in 2 cases, whilst an overestimation was seen in 5 cases. One patient was also underestimated in comparison with 99mTc-PYP. 111In-AM images showed a poor quality, with considerable liver, bone marrow, kidney and blood pool activity and therefore low target to background ratio. In-vivo kinetics was characterized by a slow clearance from the blood pool.(ABSTRACT TRUNCATED AT 250 WORDS)