The platelet-derived growth factor receptor-beta (PDGFR-beta) has a number of conserved cysteine residues on its cytoplasmic domain. We have examined whether the cysteine residues play a role in the enzymic function of PDGFR-beta. We found that N-ethylmaleimide, which selectively alkylates free thiol groups of cysteine residues, completely inhibited the kinase activity of PDGFR-beta. We then identified, through site-directed mutagenesis, two conserved cysteine residues critical for the enzymic function of PDGFR-beta. Cys to Ser mutations for either Cys-822, positioned in the catalytic loop, or Cys-940, located in the C-terminal kinase subdomain, significantly reduced the activities of autophosphorylation and phosphorylation towards exogenous substrates. The non-reducing gel analysis indicated that neither of these cysteine residues contributes to the kinase activity by disulphide-bond formation. In addition, the individual mutation of Cys-822 and Cys-940 had no effect on protein stability or the binding of substrates or ATP, implying that these cysteine residues are involved in enzyme catalysis. Finally, proteolytic cleavage assays showed that the mutation of Cys-940, but not Cys-822, induced a protein conformational change. Taken together, these results suggest that Cys-940 contributes to the catalytic activity of PDGFR-beta by playing a structural role, whereas Cys-822 contributes through a different mechanism.