The response of xenografts of five human malignant glioma cell lines and two human squamous cell carcinomas to fractionated irradiation was studied. For this, the tumors were transplanted into nude mice which had been further immunosuppressed by 6 Gy whole-body irradiation. Radiation was given as 30 fractions applied under normal blood flow conditions in two sessions per day over 15 days. Absolute and specific tumor growth delay after 48 Gy, and tumor control dose 50% (TCD50) were evaluated. Using local tumor control as experimental endpoint, four out of five malignant gliomas were more resistant to fractionated radiation therapy than the two squamous cell carcinomas. The TCD50s of these four gliomas ranged from 73 Gy to more than 120 Gy, whereas the TCD50s of the squamous cell carcinomas were 51 and 60 Gy. Absolute tumor growth delay correlated well with TCD50, but no correlation was obtained between specific growth delay and TCD50. The response of the human tumor xenografts in vivo did not correlate with the surviving fractions at 2 Gy of the same cell lines in vitro which have been previously obtained in our laboratory. The results suggest that the unique radioresistance observed in malignant gliomas in patients is at least in part reflected in human tumor xenografts. The lack of correlation between the surviving fraction at 2 Gy in vitro and the tumor response in vivo could be a consequence of an immune response by the host, a difference in cell radiation sensitivity between cell lines and xenografted tumors, or of differences of parameters such as hypoxic fraction, rate of repopulation, and cell cycle effects between the different tumor lines studied. It illustrates the difficulties which might be involved in the prediction of the response of individual tumors to radiation therapy based solely on the intrinsic radiosensitivity of the tumor cells as assayed by in vitro assays of colony formation.