Recently, there has been a growing body of opinion that tumour cells progress from a less malignant to a more malignant (metastatic) phenotype, due to an inherent instability within the genome. It has also been suggested that genomic instability and the rate of generation of metastatic variants both increase as the tumour cells achieve a higher state of malignancy. In this review, several different aspects of genomic instability have been discussed with particular reference to the low (F1) and high (BL6, ML8) metastatic variants of a B16 murine melanoma. The induced mutation frequency, gene amplification, and sister chromatid recombination correlated with the order of metastatic potential. Survival data for x-irradiation and bleomycin were similar between the three cell lines. When these agents were used to induce strand breakage, no difference was detectable in either the rate or extent of DNA single or double strand break repair. The fidelity of double strand break religation was measured using the PMH16 plasmid, and the frequency of homologous recombination was measured using the DR plasmid. Surprisingly, both of these parameters showed a correlation with metastasis which was opposite to the expected result, with regard to the concept of genomic instability. A basis for this might be the functional heterogeneity within the cell lines in respect of the properties measured. Therefore, some aspects of instability may have to be measured at a more critical level if they are to be of value in estimating the inherent instability of the genome.