Injuries to ligaments and tendons occur very commonly, but often heal very poorly. A number of growth factors show promise as biological agents with which to augment the reparative responses of these tissues, but they are extremely difficult to deliver clinically. We have suggested the use of gene transfer as a possible way to obviate this limitation. Here we have used the lacZ marker gene to evaluate methods for gene delivery to ligaments and tendons. Cell cultures were established from the following tissues of the New Zealand white rabbit: anterior and posterior cruciate ligaments, medial collateral ligament, patellar tendon and semitendionsus. In vitro experiments confirmed that these cells were very efficiently transduced by an E1- E3- adenoviral vector carrying the lacZ gene. Cells derived from ligaments and tendons also proved readily susceptible to retroviral transduction and, following transfer of the ned gene, selection in G418. To determine whether these cells could be genetically altered in situ, gene transfer to the patellar tendons of rabbits' knee joints was attempted using an adenovirus for in vivo delivery and a retrovirus for ex vivo delivery. Direct, percutaneous injection of the adenovirus resulted in high levels of lacZ expression, but this was limited to the area of tendon immediately adjacent to the injection site, and preferentially labeled cells in the epitenon sheath. An unexpected finding of these studies was the in vivo adenoviral transduction of bone cells in areas where the tendon insertion site had been accidentally injected. Allotransplantation of tendon fibroblasts into which the lacZ gene had been retrovirally introduced led to lacZ expression throughout the body of the tendon itself. The genetically altered cells appeared able to migrate from the site of injection and to integrate themselves into the crimp pattern of the tissue. Both in vivo and ex vivo methods led to gene expression for at least 6 weeks, which may be long enough for clinical purposes.