An effect of electrostatic fields on the chromosomes of Ehrlich ascites tumor cells exposed in vivo has been demonstrated. Cells exposed to horizontal electrostatic fields for two weeks had almost a threefold increase in the percentage of abnormal chromosomes when compared to control cells or cells exposed to vertical electrostatic fields for the same period. Extended exposure times of 4--15 weeks resulted in the disappearance of the aberrant chromosomes. It is suggested that the effected cells were incapable of cellular replication resulting eventually in their disappearance via cell death.