Inflammatory cytokines have been implicated in the pathogenesis of rheumatoid arthritis. To validate a key role for IL-1 in arthritic processes we have studied the protective effect of neutralizing antimurine IL-1 antibodies in the murine collagen-induced arthritis (CIA) model. Combination of anti-IL-1 alpha and anti-IL-1 beta given before onset of arthritis was shown to prevent disease completely. Remarkably, a single treatment was also highly effective in the established phase of arthritis, reducing both inflammation as well as cartilage destruction. Suppression was most pronounced with the combination, but anti-IL-1 beta alone also induced significant relief. Finally, we studied the protective effect of IL-1 neutralization on cartilage metabolism in a unilateral expression model of collagen arthritis. To this end zymosan was injected in one knee joint before onset of disease, resulting in accelerated expression in that particular joint and the draining paw. Anti-IL-1 treatment started after accelerated expression of arthritis was able to fully normalize chondrocyte synthetic function, which was highly suppressed in the control group. It is concluded that IL-1 is an important determinant in both inflammation and cartilage destruction in collagen arthritis, and this may have implications for therapy in human arthritis.