Human blood has been irradiated with alpha-particles from an external source of curium-242. The collimated alpha-particles entered the blood with an energy of 4·9 MeV and were almost completely absorbed by the blood. After culturing for 48 hours, the dicentric yield in the lymphocytes at the first metaphase was measured as a function of dose to the blood. The yield was linear with dose up to 400 rad with a slope of 28·6 × 10-4 dicentrics/cell per rad. This is equivalent to an initial slope r.b.e. of 17·9 with respect to cobalt-60 γrays. This value disagrees with the only two other published values in the literature. Reasons for this disagreement are discussed. Compared with neutron r.b.e. values obtained in this laboratory the alpha-particle values we observe are surprisingly low. A model is proposed which predicts low values of r.b.e. for chromosome aberration production using radiations of high LET. The low values occur because there is a distribution of specific energy between cells which causes a selective removal of cells likely to contain higher numbers of aberrations.