Immunologic similarities have been demonstrated between Coxsackievirus B3 (CVB3), group A streptococcal M protein, and cardiac myosin. Previous studies have also shown that T lymphocytes obtained from CVB3-infected mice expressing the H-2k MHC haplotype gave an immunodominant proliferative response to the NT4 peptide (GLKTENEGLKTENEGLKTE) of the streptococcal M5 protein. We now show that the NT4 peptide can induce inflammatory heart disease in MRL/++ (H-2k) mice and that induction of anergy to this peptide protects against CVB3-induced myocarditis. MRL/++ mice infected with CVB3 for 7 days or immunized twice at 7-day intervals with the streptococcal NT4 peptide in CFA developed myocarditis. Treatment of the immunized mice with either anti-CD4 or anti-IAk mAbs inhibited cardiac inflammation. Injection of MRL/++ mice with NT4 covalently coupled to syngeneic splenocytes tolerized the animals to this peptide as shown by reduction of the proliferative response. NT4-tolerized mice had significantly reduced myocarditis, although virus titers in the heart were elevated. A control peptide, VP1-10 from the CVB3 capsid protein VP1, did not protect the mice from CVB3-induced myocarditis. The results suggest that immunity to NT4 induced during CVB3 infections is important to the development of cardiac inflammation.