In this paper the direction of transverse drift of those actively forming osteons which show drift was quantitated with respect to: (1) per cent drifting toward the marrow cavity, (2) per cent drifting toward periosteum, and (3) per cent drifting intermediately. It was found that approximately 71 per cent of all drifting osteons drift toward the marrow cavity. This value was not related to age or sex. On the basis of these results it was postulated that some agent in the marrow cavity favors a negative cortical endosteal bone balance, that some agent at the periosteal surface favors a positive periosteal bone balance, or that both factors are involved.