To determine if articular cartilage collagen fiber organization differs with location on the tibial plateau, specimens from dogs, humans, and rabbits were studied by scanning electron microscopy. Joint surfaces were fixed, dehydrated, and fractured radially so that the periphery could be compared with the center on single specimens. Generally, fibers were more tightly packed in the lateral side than in the medial and in the periphery as compared with the center, where the cartilage was consistently thicker and the radial zone was dominant and composed of straight vertical fibers. In the periphery, the tangential and transitional zones were better developed and contributed up to 50% of the cartilage depth in comparison to only 5% centrally. The soft, dull, malacic appearance of the center results from lack of a true surface layer of tangential collagen fibers.