Immunological identification of lymphocyte subsets in a patient with myocarditis revealed an increase in myocardial OKT 11 (pan T), OKT 4 (inducer/helper T) and OKT 8 (suppressor/cytotoxic T) subsets associated with a transient decrease in the percentage of circulating OKT 3 (pan T) and OKT 4 (inducer/helper T) subsets. This decrease may be explained by the accumulation of these subsets in the diseased myocardium. Specific antigenic markers on lymphocytes at the site of myocardial inflammation in the acute stage of myocarditis differ from those on corresponding peripheral lymphocytes. This observation may highlight the immuno-pathogenetic mechanisms involved in the development of myocarditis.