Sheets of MDCK cells were severely damaged by the commonly used antiseptic, chlorhexidine. Damage was indicated by changes in the characteristic blister formation normally observed in sheets of these cells growing on the base of culture flasks. Results are given for reduction in blister number with increasing chlorhexidine concentration. Damage was indicated also by increasing transport of radiolabeled inulin across the sheets after treatment with increasing concentrations of chlorhexidine. The mechanism of chlorhexidine damage is discussed. The conclusion is that chlorhexidine could be an important contributory factor in the failure of ultrafiltration in peritoneal mesothelial cells, and its use should be avoided with magnesium-containing dialysis fluids in kidney-failure patients.