Six rheumatoid articular cartilage specimens, which appeared grossly normal and were shown to be free of pannus when examined under the light microscope, were examined electron microscopically. For comparison, normal-appearing cartilage specimens from 2 patients with meniscus injury and 2 with degenerative joint disease (DJD) were also examined. In all cases the normal-appearing joint surface of rheumatoid cartilage was abnormal. Amorphous-appearing material was present to a depth varying between 6 and 25 micron. Some of this material had the appearance of fibrin deposited at the cartilage surface, but much appeared to represent breakdown products of the cartilage matrix, i.e. degraded collagen and proteoglycan. DJD cartilage did not show similar changes. The findings suggest that the surface of rheumatoid articular cartilage, even when grossly normal in appearance, is degraded by enzymes either present in the synovial fluid or released by polymorphonuclear cells in close contact with the cartilage surface.