Five permanently established xenograft lines of human soft tissue sarcomas were irradiated with single doses of 5.8 MeV d(14)+Be neutrons and of 60Co rays, respectively, at several dose levels to generate dose response relationships. The tumors were clamped ten minutes prior to and during irradiation to induce uniform hypoxia. All tumours were previously characterized by means of histomorphology, tumour doubling times (DT's), DNA-index and enzyme pattern of the lactate dehydrogenase (LDH) and glucose-6-phosphate dehydrogenase (GPD). According to these criteria, three out of five leiomyosarcomas were identical referring to the biopsy of origin, whereas two had changed in successive passages. For the different tumour lines, specific growth delays ranged from 0 to 8.7 after 5.3 Gy neutrons and from 0 to 11.4 after 16 Gy60Co, respectively. In terms of radiosensitivity for different single doses and irradiation qualities, a highly significant overall correlation (rs = 0.82 +/- 0.06) was found for the ranking of the tumours with respect to the growth delay and specific growth delay endpoints. No correlation was found between tumour doubling times and the relative biological effectiveness (RBE). In general, calculated RBE-values decreased with increasing effect level. For the five tumour lines, RBE-values ranged from 1.6 to 12.7 and 2.0 to 4.4 at specific growth delays of 0.5 and 2.0, respectively, under acutely hypoxic conditions. These results indicate a potential advantage for neutrons in a subgroup of human soft tissue sarcomas compared with sparsely ionising irradiation.