The downregulation of tyrosine kinase receptors attenuates signalling and is thought to be dependent upon intrinsic receptor kinase activity, largely because downregulation is inhibited by a kinase-inactivating mutation of an invariant lysine residue of the receptors for EGF, insulin, M-CSF and PDGF. We con®rmed that this mutation inhibited the degradation of the M-CSF receptor. However, two di erent kinase inactivating mutations of the invariant amino acids Gly 591 and Glu 633 did not prevent M-CSF-induced receptor degradation, so demonstrating that receptor kinase activity is not essential for this process. Three other kinase-inactivating mutations were found to cause constitutive receptor degradation in the absence of M-CSF, most probably by disrupting the structure of the activating loop of the kinase domain. It is known that extensive movement of the A-loop is necessary for kinase activation and is normally induced by ligand-binding. It is therefore suggested that some aspect or consequence of the change in structure of the A-loop caused by ligand binding also activates receptor downregulation, so ensuring that downregulation is coupled to but is not necessarily dependent upon receptor kinase activity.