Ultrastructural changes of knee articular cartilage in C57B1/6 mice were studied in the course of antigen-induced arthritis by means of scanning and transmission electron microscopy. First damages of the cartilage surface were seen one hour after arthritis induction. The earliest signs were the loss of the superficial electron-dense layer as well as a progressive loss of proteoglycans in the cartilaginous matrix on the surface. Breaks of collagen fibres were already detected at the first day of arthritis. The chondrocytes of the superficial cartilage layer showed an increase of the intracellular membraneous system in the early phase of arthritis. Thereafter chondrocytes on the surface became more and more necrotic. On the 7th day of arthritis acute alterations of cartilage had developed completely. Many lacunae of chondrocytes were opened and the cartilage surface showed deep structural defects with adhering cells, probably lymphocytes and macrophages. In the following time these destructive processes were demonstrated along with cellular proliferation as a sign of repair attempts in hyaline cartilage.