If the newborn of a socially organized CBA/USC mouse colony are regularly removed (force breeding), the social order breaks dowm. There is fighting among the males and the young are lost due to neglect and injury by the females (cronism). A high incidence of mammary tumor formation is observed during the disorder of such formerly socialized groups. In the pilot study being reported, force breeding initiated at the sixth month of colony life eventually at the ninth month led to fighting and the loss of young by cronism. All 12 female colony members developed tumors during the subsequent 5 months. Meanwhile, tumors developed in only 8% of the same age Study Siblings and in 46% of Study Breeders maintained under rapid breeding conditions. Force breeding is a knowm moderately effective tumorigenic technique. Since tumors did not occur in the population cage until its social system broke down, this suggests that the combination of force breeding and social disorder-induced neuroendocrine changes is peculiarly favorable to tumor development.