The idea that one alpha particle (with LET approximately 100keV/micrometers) traversing a cell nucleus would kill a cell has been a concept which has been traditionally accepted by most radiation biologists. It was, therefore, difficult to see how alpha radiation could act directly on nuclear DNA to cause cancer. In experiments where mouse embryo cells (C3H 10T1/2) were irradiated with a parallel beam of 5.6 MeV alpha particles from a Tandem Van de Graaff machine, we made the surprising discovery that about 10 to 20 alpha particles through each nucleus were required to kill 2/3 of the cells (Lloyd 1979a). Earlier workers were misled because they failed to make measurements of nuclear areas of the cells as they were irradiated. We have used scanning electron microscopy, both to observe the cells at the time of irradiation and to document the change in unirradiated cellular dimensions as the cells become flattened on conventional tissue culture plates. The present paper also describes the morphological changes observed by SEM between cells which were subsequently transformed to become malignant by alpha irradiation and untransformed control cells.