The potentiation of radiation damage by hadacidin in proliferatingE.colihas been investigated. Apparently this effect of hadacidin may be due to an effect in previously described mechanisms of inhibition by hadacidin: (a) inhibition of adenylosuccinate synthetase and (b) undefined inhibition of energy production.E.colicells grown in the presence of adenylic acid, adenine, oxalacetic acid, or anthranilic acid for 4 hours before exposure to radiation are insensitive to the hadacidin radiation effect.