AbstractThe induction of Lambda prophages in lysogenicEscherichia colicells is started after inactivation of repressor molecules coded for by the prophage genome itself. All agents known to inhibit bacterial DNA synthesis are able to initiate a set of processes resulting in the inactivation of these repressors. In the case of UV light as an inducing agent it is suggested that the UV mediated damages within the bacterial DNA diminish the replication velocity leading through a set of intermediate processes to the inactivation of phage repressors by accumulation of repressor inactivating precursors of DNA synthesis. The analytical function for the dose effect curve is obtained from the product of the probabilityPIof cells containingIUV damages multiplied by the numberIof UV damages after summing up all possible numbers ofI. The probability ofPIis calculated on the basis of the radiation kinetic model ofRONTÓet al.(1972). The theoretical dose effect curve, obtained shows a shoulder with an extrapolation number of 1.2 and a linear part at high irradiation doses in semilogarithmic plot. The experimental data obtained with UV induction ofE. coliK12 (λ) are consistent with the model and show that only a few UV damages smaller than 10 per bacterial chromosome are sufficient for inductio