Abstract: Present invention relates to a composition for use in the treatment of arthritides. Alkaline phosphatase (AP), an ectophosphatase with an anti-inflammatory and barrier protecting mechanism of action shows potent anti-rheumatoid arthritis (anti-RA) efficacy in a rat model for arthritis. In this model RA was induced by subcutaneous immunization with a mixture of methylated bovine serum albumin (mBSA), CFA (complete Freund's adjuvant antigen) and CBP (custom Bordetella pertussis antigen) and intra-articular injections of mBSA. Results were comparable with those obtained for MTX, the drug reference compound for the treatment of RA. Both knee swelling over time and the number of invading macrophages were found to be reduced with AP treatment, either applied in prophylactic treatment or therapeutic treatment, and comparable to the MTX effects. AP was found effective both as prophylactic, and as therapeutic intervention.