Monoarticular arthritis in the rabbit has been used to study the effect of intra-articular administration of cortisol-21-phosphate and alpha-1-proteinase inhibitor. The preparations were administered both separately and in combination. All treatments improved parameters associated with joint biochemistry and histopathology, but the greatest effect was found when steroid was combined with anti-proteinase. Cortisol-21-phosphate had both an anti-inflammatory and anti-arthritic action, whereas alpha-1-proteinase inhibitor showed little anti-inflammatory action but had some anti-arthritic effect. Alpha-1-proteinase inhibitor had no anti-inflammatory action against carrageenan induced oedema in the rat, but was anti-arthritic against adjuvant induced arthritis in the rat where it reduced both primary and secondary arthritis.