The HER4/ERBB4 gene encodes a 180K transmembrane protein (HER4/p180erbB4) that is structurally related to the 185K product (HER2/p185erbB2) of the HER2/ERBB2 proto-oncogene. A 45K heparin-binding glycoprotein (p45) has been characterized that specifically activates the intrinsic tyrosine kinase activity of HER4 (ref. 2). This HER4 ligand shares several features with the heregulin family of proteins, including molecular mass, ability to induce differentiation of breast cancer cells, activation of tyrosine phosphorylation in MDA-MB453 cells, and amino-terminal protein sequence. Heregulin exists as multiple isoforms and all are presumed to interact directly with HER2 (refs 3-6). We have used binding and phosphorylation studies with recombinant ligand on cell lines expressing recombinant receptors, and report here that heregulin, like p45, is a specific ligand for HER4. Furthermore, heregulin fails to induce phosphorylation of HER2 in the absence of HER4. These findings suggest that activation of the HER4 receptor is involved in signal transduction by heregulin.