Experiments were designed to compare specific and nonspecific resistance to induced bacterial disease in mice placed at 2° C. The animals were acclimatized or unacclimatized to the low temperature. It was found that specific resistance afforded the maximal protection possible whereas nonspecific resistance was of relatively low extent. The low ambient temperature did not interfere with protection gained from specific immunization provided the cold-exposed mice were caged in small groups (10 per group) as compared to individually caged animals. The difference in results between isolated and grouped animals was not due to conservation of heat by huddling. It appears that the effect of isolation contributes to enhanced mortality following challenge.