The effect of chlorine and ozone on Escherichia coli cells resuspended in waste-water was compared. Selected chlorination and ozonation conditions produced a similar decrease in culturability (2-2·5 log). Under these conditions, differences in membrane permeability and cell surface hydrophobicity, depending on the disinfectant tested, were detected. After ozonation, while no changes in cell surface hydrophobicity were observed, approximately 95·5% of cells showed altered membrane permeability. The effect of chlorine was not linked to changes in membrane permeability. After chlorination, E. coli cells showed a tendency to aggregate. The possibility that aggregation of cells could interfere with conventional colony counts is discussed. The degree of toxicity (Microtox™ assay) was unrelated to the effect on cellular activity.