Inflammatory and noninflammatory juvenile synovial fluid (SF) samples were examined in vitro for their effect on cartilage. SF from 3 of 6 patients with pauciarticular juvenile arthritis (JA), 7 of 7 patients with polyarticular JA and 2 of 2 patients with ankylosing spondylitis had significantly increased proteoglycan (PG) releasing activity in 4 day cultures of living but not of freeze-killed cartilage. Noninflammatory SF did not cause increased PG release. Cartilage collagen content was not significantly altered by either inflammatory or noninflammatory SF. Our in vitro results showed PG releasing activity was present in inflammatory but not in normal juvenile SF.