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Tumoricidal agents have been used to kill viable exfoliated tumour cells following colorectal cancer surgery. Recent in vivo experiments have thrown some doubt on the tumoricidal activity of povidone-iodine.The cytotoxic effect of distilled water and of povidone-iodine at 0.04, 0.4, 0.8, 2 and 4 per cent final concentrations on human SW620 colonic cancer cells in the presence of red blood cells, purified haemoglobin and red blood cell (RBC) membranes, plasma, albumin, faeces and bacteria was investigated. Cell viability was assessed using the trypan blue assay and MTT test.The presence of albumin and plasma decreased the tumoricidal activity of povidone-iodine except for the highest concentration tested. Bacterial suspension did not influence the efficacy of povidone-iodine. Faecal material was found to have an intrinsic tumoricidal effect. Both intact and lysed RBCs very strongly inhibited the tumoricidal activity of all povidone-iodine concentrations tested. This inhibitory effect was due to haemoglobin, but not to RBC membranes.Low concentrations of povidone-iodine fail to kill all ‘exfoliated’ cancer cells in the presence of proteins, intact or lysed RBCs. Therefore, washing out of these organic materials before application of a relatively high povidone-iodine concentration (e.g. 5 per cent or greater) may be more useful in killing viable exfoliated tumour cells during surgery for colorectal cancer.