M-DNA is a novel duplex conformation in which metal ions such as Co² , Ni² or Zn² replace the imino protons of every base pair. An ethidium fluorescence assay was used to estimate lesions in M-DNA induced by γ- and UV radiation. General damage to DNA was assessed from the loss of ethidium fluorescence after irradiation of calf thymus DNA. Crosslinks were measured from the return of ethidium fluorescence after a heating and cooling step. Strand breaks were estimated from the loss of fluorescence in covalently closed circular plasmid DNA after a heating and cooling step. For the Co² form of M-DNA, γ-radiation caused the very efficient formation of crosslinks which was not observed with B-DNA nor with the Ni² or Zn² forms of M-DNA. The crosslinks occurred in both A–T and G–C base pairs but did not form in the presence of a free radical scavenger. Crosslinks induced by UV radiation also formed at a faster rate in the Co² , Ni² and Zn² forms of M-DNA compared to B-DNA; crosslinking occurred in all DNA but was more prominent in AT-rich sequences and was not inhibited by a free radical scavenger. Therefore, the presence of certain metal ions may lead to large increases in the formation of radiation-induced crosslinks in DNA.