To determine if immunosuppressive acidic protein (IAP) responds to the degree of inflammation in adjuvant arthritis in rats (AA), we measured the serum IAP level of the rats by using the single immunodiffusion method, and the level of concanavalin A (Con A) binding protein by using nepherometry. Pharmacologic treatment with 0.3mg/kg of methotrexate (MTX) was done from day 5 to 14 postimmunization in rats with AA. The measured serum IAP level on day 21 indicated the most severe inflamed phase. IAP levels reflected the degree of primary and secondary inflammatory phases in AA, whereas ConA binding protein (CBP) levels did not. Our results suggest that IAP can be used to monitor the symptoms of inflammation and the efficacy of anti-inflammatory drugs.