Progressive ankylosis, ank, a new recessive skeletal mutation causing a noninflammatory joint disease in mice is described. Preliminary observations suggest that three abnormal processes are involved: increased calcification of calcified cartilage and of joint tissues, hyperplasia of the cells and tissues of the joints, and degeneration within and around the tissues, tendons, and ligaments of the joints. The new mutation is closely linked, with about 4 percent recombination, to underwhite on chr 15.