1. Pig synovium in organ culture produces material which induces living cartilage to resorb its proteoglycan in vitro. 2. The bioassay for this material was to measure glycosaminoglycan released from explants of bovine nasal-septal cartilage cultured for 8 days. The performance of the assay was greatly improved by adding cortisol succinate (0.1mug/ml). This decreased the release of glycosaminoglycan from unstimulated cartilage without inhibiting its response to catabolic factors from the synovium. 3. By using this improved assay it was shown that 90% of the active materials in synovial culture medium were retained by dialysis membrane. 4. An active protein was partially purified from synovial culture medium by (NH(4))(2)SO(4) precipitation, ion-exchange chromatography, gel filtration and preparative isoelectric focusing. 5. This protein, called catabolin, had mol.wt. 17000 and pI4.6. 6. Synovial culture medium concentrated in dialysis tubing was subjected to gel chromatography and found to contain one major active component, which was eluted at the same position as the partially purified catabolin. 7. The synovial culture medium was not inactivated by heating (70 degrees C for 10min), nor were diluted preparations of partially purified catabolin, but concentrated crude preparations were thermolabile. 8. These results suggest that catabolin is the major substance produced by the synovial tissue in culture which induces resorption of proteoglycan of living cartilage in vitro. 9. Other cultured soft connective tissues produced catabolin-like activity. The example of sclera is shown, and production was inhibited by cortisol succinate (0.1mug/ml). 10. It is suggested that catabolin may be a general product of soft connective tissues in culture, and its physiological function may be to induce resorption of connective-tissue matrix after injury.