Agonist antibody is disclosed which binds to the extracellular domain of receptor protein tyrosine kinase pTK, and thereby cause dimerization and activation of the intracellular tyrosine kinase domain thereof. The antibody is useful for activating its respective receptor and thereby enabling the role of the tyrosine kinase receptor in cell growth and/or differentiation to be studied. Chimeric proteins comprising the extracellular domain of the receptor pTKs and an immunoglobulin constant domain sequence are also disclosed.
