The articular cartilage in pyogenic arthritis, induced in rabbit by intra-articular injection of Staphylococcus aureus, was studied with the electron microscope. The cartilage showed erosion of the surface and degeneration and necrosis of the chondrocytes starting from 24 hours onward, following the intra-articular injection of bacteria. The surface erosion and chrondrocyte necrosis progressed rapidly. The changes are believed to be due mainly to liberation of lysosomal enzymes from the synovial cells and polymorphonuclear leukocytes.