The authors used the blind method for evaluation of the morphological picture of the joints and the level of circulating immune complexes to study the effect of prolonged oral administration of dimethyl sulfoxide (DMSO) and its main metabolite dimethyl sulfone on the development of spontaneous arthritis in 36 Mrl/Mn/lnr female mice. It was found that DMSO and dimethyl sulfone lessen the destructive changes in the joints, while DMSO also inhibits the manifestation of immune disorders, i. e. produces a "basal" effect on the course of spontaneous chronic arthritis in experimental animals.