The effects of a monoclonal antibody (MoAb DB-1), which neutralized rat interferon-gamma (IFN-gamma), on the induction and progression of adjuvant arthritis in Lewis rats induced by intraplantar injection of Freund's complete adjuvant was studied. The animals were treated intraperitoneally with MoAb DB-1 (0.3-5 mg) for various times. Prophylactic treatment with MoAb DB-1, starting 2 days prior to arthritis induction, inhibited oedema in both the injected and non-injected hind paws and delayed joint destruction as shown by radiography. However, despite continued MoAb treatment, the disease progressed. High doses of MoAb DB-1 exacerbated the disease. A control MoAb of the same isotype did not have significant effects on adjuvant arthritis. Therapeutic treatment with the MoAb DB-1 starting 8 days after arthritis induction caused only slight and short-lived inhibitory effects. IFN-gamma appears to be a critical lymphokine for the development of adjuvant arthritis.