H uman blood mononuclear cells (BMC) in short-term culture secrete one or more factors that induce degradation of matrix proteoglycan and collagen in cartilage explants in organ culture. Induction of matrix degradation took place both in nasal septum and articular cartilage explants in the presence of the mononuclear cell supernates. Cartilage degradation in this system was absolutely dependent on the presence of live chondrocytes. Matrix depletion did not occur in dead cartilage explants cultured with active supernates. Supernates obtained from unstimulated BMC showed variable cartilage matrix degrading activity (MDA). BMC stimulated with phytohemagglutinin (PHA) showed increased MDA, which in one dilution experiment was found to be five times higher than that in the unstimulated control supernate. Concanavalin A and pokeweed mitogen were also shown to stimulate release of MDA. Time experiments showed that most of the degrading activity was released by the mononuclear cells during the first day of culture. The cellular origin of MDA was investigated with the aid of partially purified BMC subpopulations. Removal of adherent cells resulted in a decrease of MDA release. Purified T lymphocytes failed to show enhanced MDA release in spite of their ability to mount a virtually intact proliferative response to PHA. Purified adherent cells also failed to show enhanced PHA-dependent MDA release. Nevertheless, restoration of PHA-dependent MDA release took place in reconstituted cell populations containing both T lymphocytes and monocytes. These experiments suggest that MDA may be released by adherent mononuclear cells, presumably monocytes, and that the PHA-dependent increase in MDA release may be mediated by T lymphocytes. Partial characterization of MDA by gel chromatography showed one active fraction corresponding to an apparent molecular weight ranging from 12,000 to 20,000. The fraction was also shown to degrade cartilage matrix only in the presence of live chondrocytes. These results demonstrate that factors released by human BMC mediate degradation of matrix proteoglycan and collagen in intact cartilage explants through chondrocyte activation. This pathogenic mechanism may play a role in in vivo cartilage destruction in chronic inflammatory joint diseases.