In our study, the quantity of the peptides from the light chain variable domains in IgGs from patients with schizophrenia does not correlate with the proteolytic activity against MBP, whereas for two peptides from the heavy chain variable regions FQ(+0.98)GWVTMTR(m/z = 563.782+) and *LYLQMN(+0.98)SLR (m/z = 613.33+) there is an increase in activity with an increase in their concentration. The gene discussed is MBP; the disease is schizophrenia.