Histochemical and ultrastructural studies of bone-cartilage junctions from 21 rheumatoid knee joints have demonstrated the presence of both osteoclasts and chondroclasts. Significant erosions of bone and mineralized cartilage were observed in 15 specimens, and 6 showed localized erosions of unmineralized (hyaline) cartilage. Chondroclasts, defined by their close association with both mineralized and unmineralized cartilage, were morphologically and histochemically similar to osteoclasts. Our observations suggest that these multinucleate cells play a crucial role in subchondral tissue destruction, but that erosion of unmineralized cartilage is primarily the result of synovial pannus tissue.