Effects of 125I decay in DNA were investigated by measurements of strand breaks and lethal efficiencies of the decays. In bacteriophages T1 and T4, local decay effects were compared with effects of the emitted electrons by induction of both single (ssb) and double strand breaks (dsb) in the intact phage head and in extended free state DNA. Most dsbs were found to result from local decay effects whereas most real ssbs are caused by the electrons. A simple one-to-one relationship seems to exist in the phages between the decays of 125I, numbers of dsbs and lethal effects. In E. coli rec+ and recA repair of dsbs was studied in addition to lethal decay efficiencies. In rec+ more than 70% of the dsbs were repaired within 1 h at 37 degrees C. No repair was observed in recA. The probability of lethality per 125I decay per completed genome was found to be 0.37 for rec+ and 0.93 for recA cells. The number of lethal events per unrepaired dsb was found to be practically equal to unity. Unrepaired dsbs thus seem to be the primary mechanism of lethality caused by 125I decay, and all unrepaired dsbs seem to be lethal.