The articular surfaces of disarticulated knee joints from MRL/lpr and MRL/n mice, aged 4-33 weeks were examined by light microscopy (LM) and scanning electron microscopy (SEM). Light microscopy did not reliably predict SEM findings. Most of the abnormalities detected by SEM were related to surface disruption of articular cartilage. However, areas of articular cartilage covered by tightly adherent non-confluent monolayers of stallate-shaped cells with intertwinning cytoplasmic processes were observed. In these areas the integrity of the underlying cartilage matrix was disrupted, with exposure of collagen fibers. These findings suggested that outgrowth of proliferating synovial cells in the joints of arthritic MRL/lpr mice may lead to cartilage destruction. © 1992 Springer-Verlag.