Embodiments of the present invention provide for novel peptides of use for detection and/or inhibition of anti-β1-adrenergic receptor antibodies. Certain embodiments concern uses of cyclic and/or linear peptides. In other embodiments, the present invention relates to novel peptides of use in diagnostic and/or pharmaceutical compositions. Some embodiments concern diagnosing and/or treating cardiac conditions. Cardiac conditions of the instant invention can concern infectious heart disease, non-infectious heart disease, ischemic heart disease, non-ischemic heart disease, inflammatory heart disease, myocarditis, cardiac dilatation, idiopathic cardiomyopathy, idiopathic dilated cardiomyopathy, immune-cardiomyopathy, heart failure, and any cardiac arrhythmia condition.