Normal and immunized mice were subjected to acute and chronic stress of 2° C ambient temperature. The mice were challenged with varying doses of Staphylococcus aureus, strain Fritchie. Immunization offered significant protection to mice kept at 21° C and to mice that were immunized, challenged and acutely exposed to 2° C in groups. No protection was observed in mice that were immunized, challenged and exposed as individuals to 2° C. Also, immunized mice that were chronically cold exposed at 2° C were not protected against subsequent challenge and showed equivocal mortality ratios compared to the normal controls challenged under the same conditions.