Short- and long-term changes in the vertebral column and other joints of mice are induced after injection of saline solution homogenates of human chronic rheumatoid arthritis (RA) synovial tissue; and congenitally in subsequent litters. Immature mice develop abnormalities of short duration. Young adult mice develop painful, reducible kyphosis which becomes a fixed kyphotic deformity with paralysis of the legs in older adults. Generalized atrophy and degeneration of the joint structures occur in the extremities. Bony eburnation, atrophy of the cartilages and disks, with intervertebral capsule rupture and angulation, is common in advanced kyphotic lesions. From the types and sequence of the changes observed, we presume that an experimental acute and chronic rheumatoid arthritis has been induced in mice by the transmission of the active agent present in human rheumatoid arthritis synovial tissues.