The accumulation and repair of radiation-induced DNA double-strand breaks (dsbs) were determined by neutral filter elution on 20 primary cultures obtained from ovarian cancer and malignant melanoma clinical specimens. The initial frequency of DNA dsbs after exposure to 50 Gy gamma-irradiation varied greatly for the individual cultures. However, melanomas were generally more efficient than ovarian cancers in repairing these DNA lesions (mean percentage of DNA dsb rejoined after 2 h: 83 versus 62%). In 13 of 20 cultures radiosensitivity was also assessed by the Courtenay clonogenic assay. The mean +/- SD of the surviving fraction at 2 Gy (SF2) was slightly higher for melanomas (0.56 +/- 0.25) than for ovarian carcinomas (0.43 +/- 0.23). No correlation was observed between SF2 and in vitro plating efficiencies or any biological characteristics of the tumour cell population, such as proliferative activity and DNA ploidy. Similarly, we failed to find any relation between the initial frequencies of DNA dsbs and SF2 in individual tumours. In contrast, a significant and direct relationship (r = 0.86, p < 0.01) was observed between SF2 and the percentages of DNA dsbs rejoined 2 h after irradiation. In agreement with reported data on human tumour established cell lines, our results indicate that the ability to repair DNA dsbs is an important determinant for radiation response even in primary cultures of clinical tumours.