Drug resistance is generally considered to be a major impediment to successful cancer chemotherapy, yet it is generally not possible to predict the degree or timing of the emergence of tumour resistance in most chemotherapy protocols. Recent developments with the single-cell gel electrophoresis or 'comet' assay for DNA damage at the single-cell level suggest that this technique might provide a method for identifying and potentially monitoring tumour cell responsiveness to many anti-cancer agents in situ. In principle, this assay could be applied to any accessible tumour being treated with chemotherapeutic agents that cause overt DNA damage. We have investigated that supposition using several rodent and human tumour cell lines exhibiting a spectrum of resistance to the DNA strand-breaking drug, etoposide. By assessing cells grown as monolayers, spheroids and xenografted tumours in immunodeficient mice, we found that the comet assay can provide not only an index of sensitivity to etoposide, but, additionally, can demonstrate the efficacy (or lack thereof) of multidrug resistance (MDR) reversing agents for cells in vitro, and tumours in vivo.