1 Synovial cells produce a protein factor, catabolin, that induces chondrocytes to resorb their own matrix with the release of glycosaminoglycan (GAG). 2 Studies were performed to determine the effect of isoxicam, a new NSAID, on GAG production in a catabolin system utilizing tissue culture of bovine nasal septum in a medium containing pooled synovial tissue from young pigs. 3 Using fresh medium containing no synovial tissue, the release of GAG by inherent cartilage breakdown was 49.7% after 8 days culture. GAG release at 8 days increased to 83.6% by the addition of synovial cells to the culture medium, due to the action of catabolin. 4 The incorporation of isoxicam, at concentrations from 5-30 micrograms ml-1, in the porcine synovial culture medium significantly reduced the GAG release to 70.9%. 5 Isoxicam at 50 micrograms ml-1 added to fresh medium without synovial tissue had no effect on cartilage breakdown or GAG production. 6 The results of these experiments show that isoxicam is unlikely to impair cartilage metabolism and may even decrease cartilage resorption.