We presents a new biophysical model of radiation induced bystander effect applicable to low LET radiation and complementing a previously published model (Nikjoo & Khvostunov, IJRB, 79:43-52, 2003). A detailed analysis of experimental data has been performed for both the high and low LET radiations showing, contrary to the general belief, the existence of a dose response in the bystander effect at very low doses. At very low doses, the model predicts a non-linear dose response. As a corollary it is proposed that the definition of the bystander effect should be modified to take into account extra recipient cells which are hit and survived.