Rapid viability tests of the Category B agent Escherichia coli O157:H7 were evaluated after disinfection with chlorine. The metabolic activity dyes ChemChrome V6, a modified fluorescein diacetate (FDA) and 5-cyano-2,3-ditolyl tetrazolium chloride (CTC) were compared to standard plate counts. ChemChrome V6 results were obtained using a solid phase cytometer and CTC results by microscopic analysis. The water-borne bacteria Legionella pneumophila and Mycobacterium avium were also tested as positive and negative controls. Under the conditions tested, CTC provided more consistency with plate count estimates of viability than did ChemChrome V6.