This study examines the potential of tendon cells to participate in tendon repair. Explants were prepared from chicken sublimis tendons and studied in tissue culture. Under defined culture conditions, in the absence of tendon sheath, tendon cells were observed to migrate in a limited concentric fashion around the margin of a 2 mm window in the explant. When a plasma clot was present in the window, tendon cell migration and proliferation were accelerated and continued until the window was confluent with cells. Electron micrographs demonstrated that these tendon cells were fibroblasts associated with extracellular collagen fibrils. Specific biochemical analyses confirmed that these tendon fibroblasts were synthesizing collagen. This in vitro study demonstrates that tendons contain active fibroblasts capable of proliferation, migration, and collagen synthesis. These findings support the hypothesis that tendons can heal by intrinsic processes.