Correlation of a wide range of published data from irradiation experiments on damage to yeast, plant and mammalian cells has been achieved in terms of the spacing of ionising events along the charged particle tracks. Damage is optimum when the mean free path for primary ionisation along the tracks in the cell nucleus matches the mean chord length ( approximately 2 nm) through a DNA segment. This finding is common to the induction of mutations, chromosome aberrations and inactivation. Damage by direct action is found to be intra-track. Consequently there can be no dose-rate dependence. A rudimentary mathematical model of direct action including allowance for recovery is presented. A key feature is the irradiation time. Non-linear survival fractions as a function of dose can be obtained for single track action. Example results are given for acute, intermediate and chronic exposures under idealistic conditions. An interesting feature of the model is the prediction of enhanced damage by heavy particles at low doses. Absolute biological effectiveness is defined and proposals are made for a new unified system of dosimetry based on the fluence of charged particles at equilibrium and the effect cross section.