Abstract Feeding 10 ppm of Aldrin or Dieldrin to C 3 HeB Fe mice for two years shortened their average life span by two months; significantly increased the incidence of histologically benign liver tumors; decreased the incidence of pneumonia (possibly owing to less cage crowding); and had no discernible effect on the incidence of mesenteric disease, intussusception, and other lesions of less frequent occurrence.