In diarthrodial joints, bone and cartilage are structurally and functionally inseparable as shown in osteoarthritis (OA), where subchondral bone changes are integral in the disease process. By ultrastructural immunohistochemistry using polyclonal antibodies against guinea-pig bone sialoprotein (BSP), we investigated the distribution of this matrix protein at the osteocartilaginous interface in Hartley guinea-pig knees at different stages of primary osteoarthritis. Between 6 and 12 months they developed moderate osteoarthritic changes predominantly in the medial condyle, progressing to severe OA at 30 months. In all age groups BSP labeling was concentrated to the osteocartilaginous interface at a 1 micron narrow zone at the interface. In the medial osteoarthritic condyle, BSP was increased as compared with the lateral nonosteoarthritic condyle, but only at 30 months, when cartilage fibrillation correlated to BSP. Our observations suggest that altered BSP abundance may be a potential bone marker for late stage OA, while early events in bone cannot be monitored. BSP is expressed early in osteogenesis and may have a role in biological mineralization and growth. Since a sharp zone of intense BSP labeling remains at a remarkably constant level throughout life in guinea-pigs, BSP may have an important structural and/or regulating role at the interface. The protein may act as an anchor of calcified articular cartilage to subchondral bone or by regulating mineralization at the osteocartilaginous interface.