Fresh collagen fibers prepared from rabbit tendons or aponeurosis were inserted in either median or sciatic nerves of the same animal. The tissue reactions induced by the autologous fibers were compared histologically with those caused by similarly inserted silk, catgut and homograft sutures in contralateral nerves. Sham operations, passing empty needles, were performed on control nerves. The treated nerves were removed on the 5th to the 30th postoperative day, and histological sections were prepared. The insertion of the different sutures in peripheral nerves resulted in tissue reactions of varying types and degrees, depending on the material used. Catgut and silk thread induced extensive degenerative and inflammatory reactions which persisted for more than 4 weeks. Homograft sutures underwent rapid organization with a minor degree of degeneration and inflammation, which persisted for only a short time. Autograft fibers resulted in minimal myelin degeneration and mild inflammatory processes for 5-10 days, which was only slightly different from the regular course of healing seen in sham-operated control nerves. These intial observations suggest that sutures of autograft fibers may prove advantageous in nerv repair.