One of the clinical features in the patients with Parkinson's disease (PD) is an apparent low incidence of colds. MxA protein is an interferon-induced protein which inhibits influenza A viral infection. MxA protein has been found in association with Lewy bodies in neurons in PD patients' brains. We performed a semiquantitative mRNA analysis by reverse transcription-polymerase chain reaction using specific primers for MxA protein. When the peripheral blood mononuclear cells (PBMC) were incubated with alpha-interferon (alpha-IFN), the relative levels of mRNA from the PD patients increased equally to those of the normal controls. However, when PBMC were incubated without alpha-IFN, the relative levels of mRNA in the PBMC from PD patients showed marked lower levels overall compared with those of the normal controls. These results may mean that the induction of MxA mRNA by alpha-IFN in the PD patients is higher than in the controls and suggest that a remarkable increase of MxA mRNA may be linked to the clinical tendency to rarely catch a cold among the PD patients.