Photodynamic therapy is a tumoricidal modality that utilizes an inactive pharmacologic agent that becomes activated on exposure to visible light. Neoplasms selectively retain and accumulate photosensitizers at levels generally higher than surrounding non-neoplastic tissues. The purpose of this study was to establish a testing method for in vitro investigation of the effects of photodynamic therapy on human musculoskeletal neoplasms by examination of the sensitivity of these tumors to photoactivation. Three human musculoskeletal neoplasms were cultured, exposed to the photosensitizer Photofrin, and then studied for their response to photodynamic therapy after laser activation. Giant-cell tumor, dedifferentiated chondrosarcoma, and osteosarcoma were examined with use of strict experimental controls. The photoradiation conditions during photodynamic therapy were kept constant. Cell viability was determined as a function of energy dose. We concluded that the three musculoskeletal tumors were susceptible to in vitro photodynamic therapy and the test system was reproducible. The optimal in vitro nontoxic incubation concentration of Photofrin was 3 micrograms/ml. A differential cytotoxic response to photodynamic therapy was exhibited by the musculoskeletal neoplasms as a function of increased dosages of energy.