Decisions made before reconstruction of the ACL include patient selection, choice of operative procedure, and selection of proper reconstructive material. Research in the use of allograft material for reconstructive grafts is of great interest for many reasons, including the advantage of not violating the patient's normal anatomy for donor tissue. A group of eight patients who received arthroscopically inserted freeze-dried bone-patellar tendon-bone allograft tissue for ACL reconstruction were selected for a prospective study. These patients were matched against a group of four autograft controls. Immunologically, the lymphocyte blast transformation test was used to evaluate the host reaction to implanted donor graft. Lymphocytes drawn from each patient were tested for their reaction against a panel of human allograft tendon and goat xenograft tendon homogenates. A stimulation index of greater than 3 was considered a positive reaction. It was hypothesized that a consistent lack of antigenicity would be observed with allograft extracts and that an antigenic reaction would be observed with the xenograft extracts. Six patients (75%) showed an immunologic reaction to the goat xenograft, and four patients (50%) showed an immunologic reaction to at least one human allograft test cell after allograft insertion. One patient had such an intense clinical immunologic response that she required removal of the allograft. All four of the autograft controls had a nonimmunogenic response to testing, as had been expected.(ABSTRACT TRUNCATED AT 250 WORDS)