1. The aim of the investigation was to compare the contributions of the genotype of the adipose tissue and of its environment to the obesity of the mutant mouse C57 B1/6J ob/ob. 2. These obese mice had been sufficiently often backcrossed onto the C57 B1/6J background to make alloplastic transplantation possible. 3. Pieces of either epididymal or subcutaneous fat from lean or obese mice were transplanted to a site underneath the kidney capsule of recipient lean and obese mice. The grafts were left in place for a month and then examined histologically to measure fat cell diameters from which fat cell masses were estimated. 4. The fat cells of obese donor fat decrease in size in a ‘lean environment’ to the size typical of ‘lean fat’ while the cells of lean donor fat transplanted into an ‘obese environment’ increase to the size typical of ‘obese fat’. 5. When both lean and obese fat tissue was transplanted onto opposite kidneys of the same mouse, the tissue acquired the cell size characteristic of its common host. 6. The internal environment is thus of paramount importance and inherent properties of adipose tissue are of lesser importance in determining obesity in ob/ob mice.