Chinese hamster V79 monolayers, V79 spheroids, and SCCVII murine tumours were examined for DNA damage using the alkaline comet assay and for cell killing by measuring clonogenicity following a 1-h exposure to doxorubicin, N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), 4-nitroquinoline-N-oxide (4-NQO), etoposide, or 3-amino-1,2,4-benzotriazine-1,4-dioxide (tirapazamine). Greater heterogeneity in DNA damage was evident in spheroids compared to monolayers exposed to these drugs, and cell survival was correlated with the fraction of cells which lacked sufficient DNA damage following treatment with tirapazamine or doxorubicin. Cell sorting experiments verified that subpopulations of cells resistant to DNA damage were also more resistant to cell killing. Significant heterogeneity was observed in cells from SCCVII tumours exposed to tirapazamine and etoposide, and comet DNA content was used to independently assess DNA damage to aneuploid tumour cells and diploid host cells. These results suggest that, for some drugs, the comet assay may be an effective method of identifying drug-resistant cells in solid tumours.