Antibodies that bind to the second extracellular loop of the human β1-adrenoreceptor (β1-AR-ECII) are produced by/obtainable from a host cell/hybridoma. The binding compounds/antibodies are particularly useful in determination of auto-anti-β1-AR antibodies in an in vitro cell based assay system in order to characterize and to identify auto-antibodies directed against the β1-AR-ECII in a biological sample.