ErbB4 is a member of the ErbB family of receptor tyrosine kinases. Because of a paucity of appropriate pharmacologic tools, little is known about ErbB4 functions in vivo. In response to this need, we hypothesized that a recombinant form of the extracellular domain of ErbB4 would antagonize ligand-induced receptor tyrosine phosphorylation and subsequent downstream signaling and could be used to probe ErbB4 function. Indeed, we show here that one such ErbB4 protein, secErbB4-26/549, is a potent inhibitor of ligand-induced ErbB4 tyrosine phosphorylation and of ligand-induced ErbB4 coupling to biological responses. Furthermore, we demonstrate that secErbB4-26/549 antagonizes ligand-induced ErbB4 signaling by acting as a ligand sink. Thus, secErbB4-26/549 is suitable for elucidating the effects of ErbB4 ligand-induced ErbB signaling in a variety of biological contexts.