ATTENTION has been directed to skeletal variations within inbred mice strains by Gruneberg1 and by McLaren and Michie2. The latter workers have surveyed most of the available substrains of C 3 H mice and have found that they fall into two general categories having either five or six lumbar vertebrae. In the course of studies on the response of liver β-glucuronidase to hormone injections, it has now been found that there are two quite distinct levels of this enzyme in certain C 3 H substrains, and that this difference represents a genetical variation in the C 3 H strain.