Transplanted neuroblastomas were subjected to altered neural influences and imposed gravitational and magnetic fields, either separately or in combination. In each case impaired vascularity, and greater extravasation of blood with associated tumor necrosis occurred in the experimental animals. The combination of both hypergravity and alternating magnetic field resulted in definite macroscopic but less marked microscopic changes than with either force alone. Neuroblastoma cells in vitro exposed to a magnetic field showed no definite growth changes, suggesting an effect of the field forces on the host. Exposure of a unicellular organism, not requiring a host for survival, to a similar magnetic field, did result in distinct cell division changes.