1. Osteogenic induction across a Millipore filter barrier by normal mouse calvarias and mouse osteosarcoma was demonstrated in mice.
2. The amount of external bone induction was considerably greater with two-week and eight-week-old mouse calvarias than with newborn calvarias, but the incidence of bone induction dropped sharply with eight-week calvarias.
3. The amount of external bone induction by the mouse osteosarcoma was from fifteen to seventy-five times greater than that by the normal bone.
4. External induction did not occur with any chamber in which bone was not growing or with any chamber containing rat calvaria or human osteosarcoma.