The technic of immunofluorescent staining of frozen sections was employed to explore the theory that sudden death in infancy might be due to hypersensitivity to cow’s milk. Blocks of tissue from six organs were taken at autopsy from 89 infants who had died suddenly, unexpectedly, and inexplicably, and 76 control infants who had died of recognized causes. Three of the major proteins of cow’s milk, conjugated with fluorescein, were used to identify and count cells producing or containing antibodies to these antigens. There was no significant increase in the numbers of such cells in infants who had died suddenly and unexpectedly when compared with the control infants. This observation does not support the hypothesis that these deaths result from anaphylaxis secondary to sensitization to cow’s milk.