The effects of the radiosentisizing agents iodoacetic acid (IAA) and p-chloromercuribenzoate (PCMB) on the production and repair of DNA single-strand breaks induced in unicellular blue-green alga Anacystis nidulans by ionizing radiation were studied. The number of single-strand breaks in DNA, as measured by the alkaline sucrose gradient technique, was increased by these enzyme inactivating agents. The cells are apparently capable of rejoining by themselves breaks atrributable to free radical attack as well as some produced by the direct action of radiation. The presence of radiosensitizing compounds during irradiation inhibited subsequent repair of single-strand breaks in the DNA of A. nidulans. It is concluded that radiosensitizing agents affect DNA repair in A. nidulans.