Treatment ofE. coliK12 AB1157 with a low concentration of toluene (10 μM) in buffer for as short an interval as ten minutes induced a large shoulder in the X-ray survival curve. This effect was independent of the presence of oxygen at the time of irradiation. A small decrease in the final exponential slope of the survival curve was also observed, which is attributed to chemical protection effected by a small amount of toluene remaining after removal of the bulk of the chemical before irradiation. Although the mechanism of the effect is at present unknown, the alteration in the shape of the X-ray survival curve from exponential to sigmoid suggests that changes in the cell membranes to a more fluid configuration may occur.