1. Serum thymic hormone was assayed in genetically-obese (C57B1/6J ob/ob) mice and lean controls (+/+, +/−) of the same strain.
2. The thymic hormone activity was higher in the majority of the obese animals compared with non-obese mice.
3. The number of antibody-forming cells in the spleen expressed as a proportion of the total mononuclear cells was increased in the obese mice.
4. It is suggested that obesity is associated with significant changes in the thymic hormone levels which may alter the relative proportion of lymphocyte subsets and cell-mediated immunity.