Lambda-prophage induction in repair-deficient and wild-type E. coli strains by heavy ions and gamma-rays has been investigated. The dose dependence of the fraction of induced cells has been measured and its initial slope (lambda-induction potency) determined. The induction by gamma-rays was found to be more efficient in a polA-repair-deficient strain; the value of lambda-induction potency is zero in lexA- and recA- strains. The lambda-induction potency increased with LET for wild-type cells but remained constant in the case of polA- mutant cells. It is suggested that the DNA damage triggering the lambda-prophage induction in the case of ionizing radiation could be a type of DNA single-strand break with complex structure which cannot be repaired by fast repair processes, and which requires a substantial level of energy deposition for induction in a DNA molecule.