To determine the site of mineralization in calcifying tendinitis, we studied the ultrastructure of tendon fragments that were obtained during surgical interventions on ten patients with that condition. We found that calcification was occurring extracellularly and essentially in areas where the tendon had undergone cartilaginous transformation. Calcific crystals were present in matrix-vesicle-like structures that were seen either singly among collagen fibers or in aggregates. When they were in aggregates, the calcifying vesicles tended to fuse with each other. Thus, the process of mineralization in calcifying tendinitis resembled those calcifying conditions that have already been described in man and animals, both normal and pathological, in which the apatite crystals are initially localized in matrix vesicles.