Summary A correlation between the mean single survivor time and percentages of survivors at a series of shorter contact times as estimated by serial dilution counts has been established.

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Washed suspensions of Escherichia coli required much shorter exposure to heating at 57° for sterilisation than unwashed suspensions, the greater resistance of which was due to materials carried over from the culture medium. The heat resistances of washed suspensions resuspended in solutions of electrolytes differed from those in non‐electrolytes. The mode of protection is discussed in terms of the charge and permeability of the bacterial surface. Varying methods of culture and periods of growth markedly affected the heat resistances of washed suspensions. Aeration, exclusion of air, presence or absence of carbon dioxide and the surface moisture of solid media all influenced the heat resistance. Washed suspensions of E. coli, on storage, retained heat resistance poorly, in contrast to retention of viability and resistance to bactericides and bacteriostats. Killing of bacteria by moist heat causes leakage from the cells of substances which will support growth of unexposed E. coli, but which will not support growth of survivors. Growth is also supported by killed cells themselves as distinct from substances leaking from the cells.

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S U M M A R Y :Suspensions of Escherichia coli I, consisting of washed cells suspended in a phosphate buffer solution, maintained a higher viability and resistance to phenol than suspensions either of unwashed cells or of washed cells suspended in water. When stored for 5 weeks at room temperature, variations in their extinction times on exposure to aqueous phenol solutions were not significantly greater than variations with suspensions freshly prepared for each determination. Loss of resistance of a stored suspension to phenol, roxenol, lysol and potassium laurate was roughly parallel. Conditions of culture of the bacteria influenced the survival of suspensions, hut the results indicated that pronounced differences may only be found in suspensions prepared from young cultures. The use of stored suspensions in the routine evaluation o f bactericides is recommended.