Cryostat sections of endomyocardial biopsies from 53 patients (mean age 41 +/- 5 years, 38 male and 15 female) clinically indicated to suffer from myocarditis were stained using monoclonal antibodies against subpopulations of T-lymphocytes and macrophages and with polyclonal rabbit-anti-human sera marking two calcium-binding proteins expressed by monocytes and macrophages appearing in inflammatory sites only. No inflammatory infiltrate cells were found in 13 cases (25%). Mononuclear cell infiltrates were present in 40 cases (75%). Ten biopsies showed a predominance of macrophages bearing the marker 27E10, characteristic for an early acute inflammation and 18 biopsies contained 25F9 positive macrophages, characteristic for a late stage of inflammation. An intermediate type of inflammation with both macrophage types present was found in 12 patients. Patients with immunohistologically confirmed myocarditis had atrial, ventricular or combined forms of arrhythmias (78%), scars in the vectorcardiogram (100%) and radiological evidence of cardiomegaly (36%). In conclusion, typing and endomyocardial biopsies for macrophage subpopulations is a sensitive new approach to assess the diagnosis of myocarditis.