The effects of cartilage and synovial fluid components such as proteoglycans, chondroitin sulphate, hyaluronic acid, phospholipids, and albumin on the growth kinetics of monosodium urate monohydrate (MSUM) crystals were investigated. MSUM seed crystals were added to supersaturated sodium urate solutions, and the rate of decrease in the concentration of growth medium was used as a measure of the growth rate. A second order dependence of growth rate on supersaturation was found, and growth rate constants were determined with an integrated form of the growth equation. The additives, hyaluronic acid, proteoglycan monomer and aggregate, and phosphatidylserine, had no significant effect on the growth rate constant. Chondroitin sulphate and phosphatidylcholine increased the growth rate constant, possibly by promoting further nucleation in the growth medium. Albumin significantly inhibited MSUM crystallisation. The possible implications of these findings on in vivo MSUM crystallisation are discussed.