In order to elucidate the role of LET in radiobiological phenomena and to provide a better scientific foundation for high LET radiotherapy, it is proposed that cultured mammalian cells be irradiated with molecular ions, particularly D plus 2. In these studies cells supported on thin foils will be traversed by closely spaced trajectories of proton pairs. The effect of separation on RBE and OER will be observed in order to determine the role of microdosimetry at the nanometer level.