This study analyzed the histological and biochemical responses of intervertebral disc tissue to intradiscal injection of varying amounts of chymopapain. To determine the appropriate amount of chymopapain needed to accomplish effective degradation of proteoglycans (PG) in the nucleus pulposus of intervertebral discs. Chymopapain is an accepted treatment alternative for patients with disc herniations. The recommended clinical dose of 2,000-4,000 pKats per injection is derived from early animal studies and empirical results in man. A lower effective dose could reduce the complication rate while providing similar clinical results. Twenty to 4,000 pKat of chymopapain was injected into rabbit discs, and the level of keratan sulfate (KS) epitope in serum was measured at different times after the injection. The animals were killed after 6 days and the injected and two neighboring discs were examined histologically. The serum KS level did not change appreciably after injection of 20 pKat, rose moderately at 100 and 200 pKat, and rose strongly at 500 pKat. Doses greater than 500 pKat did not result in further increase in the KS level. Degradation of the disc proteoglycans is dose dependent and reaches a maximum at 500 pKat. Higher doses appear not to cause further loss of aggrecan molecules, and injection of more than 1,000 pKat produces significant annular destruction.