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A small virus resembling parvoviruses in its morphological and physicochemical properties was derived from synovial tissue of a patient with severe rheumatoid arthritis. This virus, designated RA-1, elicits a syndrome in neonatal mice that includes neurological disturbances, permanent crippling of limbs, dwarfism, alopecia, blepharitis, "masking," and a rigid curvature of the thoracic spine. Polyclonal antibodies against RA-1 display high virus neutralizing activity and in immunoassays detect reactive antigen in synovial cells from different rheumatoid arthritis patients but not persons with osteoarthritis. Putative parvoviruses isolated from several other rheumatoid arthritis patients are only weakly pathogenic for newborn mice but can generate RA-1 virus-specific antigens in tissues of these animals. It has not been established that RA-1 and existing parvoviruses of mammalian species are related.