Objective and Design: To investigate whether mast cells (MCs) and chymase, the major protease of murine MCs, were involved in a chronic fibroproliferative disorder of the paws associated with type II collagen (CII)-induced arthritis.¶Materials: Eighteen DBA/1J mice were divided into 3 groups and were used to study fibroproliferative changes in paws elicited by immunization.¶Treatment: Arthritis was induced by immunization with CII, which was intradermally injected as an emulsion made with adjuvant. A booster shot was done 3 weeks after the initial shot. A group with no treatment and that received adjuvant alone served as control.¶Methods: Twelve weeks after the booster shot, inflammation of the paws was evaluated for pathological and biochemical indices. Chymase activity was determined with a chromogenic peptide substrate.¶Results: In CII-immunized group, collagen bundles accumulated around the destructed joints. In accordance with the pathological findings, MC density in the affected paws was increased (154.8 ± 13.3/mm²; p < 0.05 vs. control) and chymase activity was also increased (29.5 ± 2.8 mU/mg protein; p < 0.01 vs. control).¶Conclusions: The present results demonstrate increases in MCs and chymase in fibroproliferative paws of collagen-induced arthritic mice.