Type IX collagen and its collagenous fragments are potent stimulatory agents on human blood mononuclear cells for the production of prostaglandin E2 and interleukin 1/mononuclear cell factor. Type IX collagen is 2-4-fold more potent that type I and II and 1 alpha-, 2 alpha- and 3 alpha-collagens. This property may be important in the destructive process of cartilage in inflammatory diseases.