Antigen-induced arthritis was established in the mouse by immunizaiton with methylated bovine serum albumin (mBSA) in complete Freund's adjuvant with B pertusis vaccine. The knee joint was injected after 21 days with mBSA in saline. The arthritis was chronic, antigen-specific, and T-cell dependent in hypothmic nu/nu mice. C57BL and balb/c mice were susceptible, whereas CBA mice were relatively resistant. Susceptibility was dominant; one gene was loosely linked to the "b" allele of the H-2 complex of C57BL mice.