Skin fibroblast cell strains and tumour cell lines were established from 12 patients with various types of soft tissue neoplasms, and radiation survival curve parameters were measured in vitro. Soft tissue sarcoma cells were consistently more sensitive to X-irradiation than fibroblasts isolated from the same patient, and were also more sensitive as a group than cell lines derived from 34 other human tumours. There was a general correlation in radiosensitivity between fibroblasts and tumour cells derived from the same patient, indicating that some component of tumour cell sensitivity may relate to genetic factors in the host. Such genetic factors, however, do not explain all of the heterogeneity in tumour cell response. The response of soft tissue sarcoma in vivo may be dependent on complex radiomodifying factors other than inherent radiation sensitivity, thus making it difficult to predict clinical outcome by use of assays which use survival of irradiated tumour cell lines in vitro as an endpoint.