Evaluation of endomyocardial biopsy specimens for lymphocytic myocarditis requires accurate identification of lymphocytes, a task at times difficult considering that other myocardial interstitial cells mimic lymphocyte morphology. To wit, the number of mononuclear cells present in normal (uninflamed) myocardium has remained in doubt. We studied the myocardium from hearts that were obtained at autopsy and transvenous endomyocardial biopsy specimens with monoclonal antibodies and immunohistochemical stains to determine the normal numbers and distribution of lymphocytes in uninflamed hearts. In the ventricular myocardium of hearts obtained at autopsy, total immunohistochemically marked lymphocytes averaged 3.6/sq mm, with most being T-cell marker-positive. The ratio of T-helper to T-suppressor-cytotoxic (OKT-4:OKT-8) cells was 1.44. The number of myocardial lymphocytes demonstrated by immunohistochemical staining correlated well with, but was consistently less than, the number obtained by quantitative light microscopic studies on unmarked samples. Thus, the immunohistochemical technique allows for objective enumeration of cells and provides avenues for quantitation of lymphocyte subpopulations in inflamed hearts.