A comparative study of the Fab- and Fab-RF-fragments in the molecules of monoclonal IgM and IgM-RF, respectively, was performed by the spin label method. The spin label, 2,2,6,6-tetramethyl-4-dichloro-triazinylaminopiperidine-1-oxyl, was introduced into the peptide part of the protein. On the basis of the data on the temperature-viscosity dependences of the EPR spectral parameters of the resultant spin-labeled proteins, the rotational correlation time τ of the spin carrier was determined. It turned out that the reduced to normal conditions τ values for the molecules of the Fab- and Fab-RF-fragments were 21±2 and 11±1 ns, respectively. Analysis of the resultant data provides sufficient grounds for assuming that such a sharp decrease in the τ value for the molecule of the Fab-RF fragment is due to local flexibility of its globular structure, which, in turn, can determine the specific features of the IgM-RF functioning as an autoantibody.