We have tested the effect of administering H22, a hamster neutralizing MoAb to murine interferon-gamma (IFN-gamma) in collagen-induced arthritis. Mice were immunized with human type II collagen in adjuvant on day 1 and boosted with soluble collagen on day 21. H22 was administered (250 micrograms, intraperitoneally) either during the induction of arthritis (on days 0, 6, 13 and 20) or around the time of disease manifestation (on days 21, 28, 35 and 42). Control mice received either an isotype-matched non-neutralizing MoAb or saline. Both treatment regimes gave similar results. Treatment with H22 did not significantly affect the incidence of arthritis, time of onset, degree of oedema, histopathological severity, or level of anti-type II collagen IgG. However, a highly significant increase (P < 0.01) in the number of limbs affected by arthritis was observed in the H22-treated group, irrespective of whether the antibody was administered during the induction of arthritis, or during the time of clinical manifestation of disease. From these results it was concluded that anti-IFN-gamma treatment caused an increase in the number of arthritic lesions, but did not affect the severity of each individual lesion.