The subcutaneous adipose tissue of genetically obese mice (ob/ob) differs from that of lean littermates not only by virtue of its larger cells but also in its fatty acid composition; it contains a higher proportion of palmitoleic acid and a lower proportion of linoleic acid. To determine whether these differences in fatty acid composition were inherent in fat cells, subcutaneous adipose tissue from obese and lean mice was transplanted under the kidney capsules of lean and obese host mice and the fatty acid composition of the neutral lipids of the graft and of the host perirenal and subcutaneous fat was determined 1 or 2 months later. The fatty acid composition of grafts from lean donors in obese mice resembled that of the perirenal adipose tissue of the obese hosts after 1 month, with a lower proportion of linoleic acid and a higher proportion of palmitoleic acid than in lean mice. Grafts from obese mice in lean mice had fatty acid compositions which were either unchanged, partially changed or which completely resembled that of the host. The use of grafts prelabeled by feeding the donor margaric acid indicated that total lack of fatty acid turnover, rather than selective metabolic processes, was responsible for the failure of some grafts from obese mice in lean mice to acquire the fatty acid composition of the perirenal adipose tissue of the host.