The radiation response of 5 different lines of human melanoma xenografts was studied. Tumours grown s.c. in the flanks of athymic mice were exposed to single doses of 5-25 Gy and subsequently analysed with respect to specific growth delay. The variation in radiation response among these melanoma lines was almost as large as that reported for human tumour xenografts differing in histological type. The most radioresistant melanomas showed longer volume-doubling times, lower growth fractions, higher cell loss factors and lower vascular density than the most radiosensitive ones. The radiation response was not correlated to the fraction of cells in S-phase or the DNA content of the tumour cells. Cell suspensions prepared from the different melanomas, irradiated under aerobic conditions and assayed in soft agar, also showed large variability in radiation response. Specific growth delay after 15 Gy was found to be correlated to the surviving fraction measured in vitro after 6 Gy, but not clearly to the Do value. It is suggested that tumour growth characteristics in vivo as well as radiation response in vitro may be of prognostic value for prediction of radioresponsiveness of melanomas.