1. When epididymal fat bodies from starved rats are incubated for 3·5hr. at 37° in a defined medium in vitro the total clearing-factor lipase activity rises to approximately twice its initial value. 2. During the incubation period part of the tissue clearing-factor lipase activity appears in the medium. 3. Heparin, glucose, insulin, and HCO3− and K+ ions are shown to be important medium constituents.