By using monoclonal anti-myoglobin antibodies to sensitize new immunospheres, serum myoglobin (SMb) was semi-quantitatively estimated with a rapid reverse passive latex agglutination (RPLA) test. Its clinical value in the early diagnosis of acute myocardial infarction (AMI) has been evaluated in 56 patients with AMI, 7 of them being observed serially. There are 92 serum specimens of AMI, of which 19 were within 12 hours after the onset of symptoms. Meanwhile, serum specimens of 72 non-AMI patients were observed with the same method. All the specimens above were also quantitated with radioimmunoassay (RIA) and the results were compared with those of RPLA. RPLA test was negative in the specimens below 100 ng/ml (81 specimens) and positive in those above 110 ng/ml (73 specimens). For specimens from 100 to 110 ng/ml, RPLA test could be either positive (9/10) or negative (1/10). This study confirms that RPLA test is a very sensitive indicator for acute myocardial infarction. It can yield a reliable semi-quantitative estimation of SMb within a few minutes. Because of its simplicity, rapidity and practicability. RPLA test is especially convenient for use in emergency room.