For risk analysis of reclaimed water, current animal and toxicity testing may not detect subtle effects such as interactions that could contribute to complex diseases such as cancers that develop over a long period of time. There is a need for assays that can be validated against known human physiological processes. We have previously validated sensitive human cell culture assays for their responsiveness to agents that induce carcinogenesis in vivo. In this initial study we analysed the effects of three batches of reclaimed water on human colonic cells. At concentrations of up to 10-fold, they had no significant effect on the cellular markers, indicating an overall lack of biological activity. The assay has potential but needs to be refined to maximise its sensitivity.