Mice of the inbred strain MRL/MpJ-lpr/lpr are affected by a systemic autoimmune disease and a spontaneously occurring polyarthritis. To characterize the arthritis a histopathological study was performed on the joints of the four limbs and of the spinal column of 7, 16, 22 and 28-week-old animals of both sexes. Polyarthritis, the severity of which increased with age was detected in all mice. Proliferation of the synovial lining cells, already evident in 7-week-old animals, was the initial lesion. In the majority of cases infiltrates containing lymphocytes with a few plasmocytes, histiocytes, polymorphonuclear neutrophils and eosinophils were detected later on. The most pronounced changes were observed in the hind-paws, the fore-paws, the knee and hip joints, paired articulations being symmetrically involved. A pannus was seen at the most in 10% of the joints leading to limited and superficial destruction of the cartilage. Rheumatoid nodules were not seen. From 16 weeks of age deposits of unknown nature, often surrounded by phagocytosing macrophages and/or neutrophils, were observed in the articular and/or extra-articular connective tissue and in the vessels. There was a positive correlation between their presence and the intensity of the arthritis. The articular lesions in our study differ from those in rheumatoid arthritis because they lacked the specific and characteristic histological features of the human disease.