Patellar tendon (PT) allografts for anterior cruciate ligament (ACL) reconstruction are potentially immunogenic and incorporate slowly compared with autografts. Our tissue-engineering approach to improve allograft efficacy is to (1) remove intrinsic cells from the graft to reduce antigenicity and then (2) seed the graft with extrinsic cells to improve ligamentization. To remove cells, tendons were soaked in 1% extraction solutions of tri(n-butyl)phosphate (TBP) or sodium dodecyl sulfate (SDS) for various time periods (24-72 h) and rinsed exhaustively. After treatment, we measured tendon cellularity, crimp structure, and mechanical properties. Treatment with either SDS or TBP removed approximately 70-90% of the intrinsic PT cells. Mechanical properties of treated PTs were similar to those of controls, despite changes in appearance. TBP- and SDS-treated PTs were then seeded with fibroblasts and cultured for up to 2 weeks in vitro. Fibroblast proliferation was retarded on SDS-treated PTs; in contrast, TBP-treated PTs supported cell proliferation similar to that of untreated controls. Extrinsic fibroblasts were successfully cultured on the TBP-treated PTs in vitro, creating viable tissue-engineered grafts potentially useful for ACL reconstruction. These modified allografts have the potential to be developed into mechanically functional delivery vehicles for cells, gene therapy vectors, or other biological agents.