The histological appearance of cells and tissues in the reparative scar tissue which forms in the equine superficial flexor tendon following partial rupture was compared to that of normal tendon. The repair fibroblasts were found to be larger and more basophilic than the tenocytes of normal tendon, to have large vesicular nuclei and to resemble the 'myofibroblasts' described in scar tissue elsewhere. The cell to matrix ratio in scarred zones of tendon was found to be increased and the concentration of collagen in these areas was less than in normal tendon. However, the scar tissue collagen was more readily extractable and contained a different pattern of collagen types. Normal equine tendon was found to be composed almost exclusively of type I collagen whereas the scarred tendon had substantial quantities (20 to 30 per cent) of type III collagen in addition to type I. The presence of type III collagen in the scarred tendon was confirmed by indirect immunofluorescence using antibodies to purified type III collagen. These observations suggest that tendon scar tissue is not derived from proliferating tenocytes but from mesenchymal cells resting in peritendinous connective tissue or blood vessels. As a result of the presence of type III collagen, the scarred tendon is also likely to have less tensile strength than normal tendon.