A treatment agent (I) for treating damaged tendons in horses comprises living tendon cells in injectable form. Independent claims are also included for: (1) use of (I) as local injection for the treatment of tendon damage in horses; and (2) preparation of (I) comprising: (a) obtaining a fresh piece of tendon; (b) cutting the tendon along the direction of the fibers 3 mm thick; (c) placing the slices in cell culture medium in culture flasks; (d) obtaining and multiplying of the cell suspensions; (e) concentrating the cell cultures in multiple washing and centrifugation steps to obtain a cell density of 10<5> cells/ml; and (f) exchanging the cell culture medium for a sterile isotonic solution.
