The lethal effect of cold shock on Aerobacter aerogenes suspensions depended on the time of exposure to low temperature, the growth phase, the concentration of bacteria, the diluent. No death occurred when weak suspensions of susceptible bacteria (about 108/ml) in buffered saline (pH 6.5) were rapidly cooled to 0[degree] and immediately warmed to 20[degree], but loss of viability was progressive during 1 hr. at 0[degree]. Bacteria harvested from defined medium at intervals during the exponential growth phase varied in sensitivity to chilling but were more susceptible than stationary phase organisms. While growing in partially synchronized culture the sensitivity of bacteria did not increase significantly during the division lag phase. The viability of dense suspensions (about 1010 bacteria/ml) in buffered saline was little affected by chilling for 1 hr. at 0[degree] irrespective of the growth phase. A bacteria-free filtrate from a chilled concentrated suspension of exponential-phase organisms substantially protected a dilute suspension from the lethal effect of chilling. Substances found in protective filtrates were amino-acids, adenosine triphosphate and nucleic-acid constituents. When added to the diluent in which susceptible bacteria were chilled, a mixture of amino-acids afforded some protection; small amounts of adenosine triphosphate had no effect. Other substances found to protect susceptible bacteria were sucrose (0.3[image]), magnesium or calcium ions (5 x 10-3 [image]) and (to a much smaller extent) spermine (10-5 [image]). The present results support the suggestion that the lethal effect of chilling is at least partly due to interference with the functioning of a bacterial permeability control mechanism.