Two types of interleukin 1 (IL-1) have been identified and purified from pig leukocytes. These probably correspond to the two IL-1s identified in humans, rabbits, and rats. The IL-1s both cause cartilage and bone resorption and act on connective tissues cells via a common receptor. The cartilage proteoglycan degradation is due to limited proteolysis, but the enzymatic mechanism is not known. Tumor necrosis factor also has been shown to cause similar cartilage degradation and both cytokines inhibit resynthesis of proteoglycan. To date these are the only cytokines known to cause these effects on cartilage.