This invention relates to polypeptides which affect myocardial cell differentiation and organisation of cardiac muscle contractile units, assay for identifying such polypeptides, and methods for improving cardiac function by the administration of such polypeptides to patients with heart disease. In one aspect, the invention provides a therapeutic composition comprising a peptide consisting of a fragment of human neuregulin .beta.2 isoform containing the EGF-like domain, e.g., SEQ ID NO.:2, and a physiologically acceptable carrier, excipient or stabilizer.