This light- and electron-microscopic immunohistochemical study using monoclonal antibodies analyzes of the in situ lymphocyte subsets in endomyocardial biopsies from 11 patients with dilated cardiomyopathy (DCM) and three patients with idiopathic (viral) myocarditis (MYO). In the DCM patients both Leu 2a+ cytotoxic/suppressor T cells (Tc/s) and Leu 3+ helper/inducer T cells (Th/i) were identifiable in the myocardial lesions, and mean Th/i/Tc/s (T4/T8) ratio was 0.7 +/- 0.6 (mean +/- SD). In 9 of the 11 DCM patients, Tc/s were more numerous than Th/i cells, so the T4/T8 ratio was less than 1.0. On the other hand, the T4/T8 ratios varied widely in the three MYO patients; one of them had marked mononuclear cell infiltrates with many Th/i in the inflammatory foci and a T4/T8 ratio of 2.6. Immunoelectron microscopy revealed some Th/i in close contact with macrophages. These T cells in the myocardium of DCM and MYO patients appeared to be in vivo effector cells playing an important role in cell-mediated immune responses.