Biological efficiency of ion beams and neutral particles can be deter-mined by studying the number of chromosome aberrations induced in ir-radiated cells. In the present investigation, we studied Relative Biological Efficiency (RBE) and statistical distributions of chromosome aberration sinduced in human peripheral blood lymphocytes exposed to 0.23 Gy and0.47 Gy of252Cf neutrons. Due to the possible cell cycle delay, the aberra-tions were scored at two different sampling times. Data were fitted usingPoisson and Generalized Poisson distributions. In all cases, experimentallymeasured distributions significantly deviate from theoretical expectations.