In an ultrastructural stereologic study on bovine articular cartilage we found that collagen volume density increased with increasing distance from the joint surface and from the chondrocyte. These results not only corroborate previous biomechanical data of a vertical stiffness gradient, but they also suggest that the mechanical forces are unevenly distributed horizontally. On the other hand, although mean collagen fibril diameter showed large differences between the interterritorial compartments of the three zones, there was a population of slender fibrils in all zones and compartments. Since the coarser fibrils provide the high tensile strength (Nimni 1988), the role of the slender fibrils may be to enhance the deformability of the tissue. Moreover, in spite of substantial differences in mean fibril diameter, collagen surface densities were in the same order of magnitude in the territorial and interterritorial compartments, and only slightly lower in the pericellular compartments. The surface parameter may be important for specific molecular interactions. The collagen fibrils have different polarity, i.e. the direction of the fibrils appears to be parallel and antiparallel, about 50% running in each direction. This, together with the very high length/diameter ratio (Clark 1985), may indicate that each fibril is assembled by the concerted action of many cells. The characteristic properties of articular cartilage depend on interactions between its macromolecular components, and the present quantitative data form a basis for discussions on the specificity and regulation of such interactions.