A human malignant melanoma grown in nude mice was exposed to single-dose X-irradiation and the effect on the proliferation kinetics was investigated by two methods. Flow cytometric DNA analysis was performed on tumour tissue obtained by sequential fine-needle aspirations after the treatment to monitor the initial cell cycle distribution changes. The technique of labelled mitoses was used to examine the kinetics of the tumours during regrowth. The results showed that the treatment initially induced a partial synchronization of small fractions of cells accumulated in the G2 phase of the cell cycle and a dose-dependent decrease of the cell generation time due to a shortening of the G1 duration time during regrowth of the tumours. Furthermore, it was shown that the calculated values of growth fraction and cell loss factor became unreliable because the tumours contained a dose-related increasing proportion of radiation-inactivated tumour cells.