We have investigated proteinases that degrade cartilage collagen. We show that pro-inflammatory cytokines act synergistically with oncastatin M to promote cartilage collagen resorption by the up-regulation and activation of matrix metalloproteinases (MMPs). The precise mechanisms are not known, but involve the up-regulation of c- fos , which binds to MMP promoters at a proximal activator protein-1 (AP-1) site. This markedly up-regulates transcription and leads to higher levels of active MMP proteins.