Abstract Curcumin, a naturally occurring phytochemical responsible for the colour of turmeric, has shown a wide range of pharmacological properties including anti-inflammatory, anti-tumour promoter and anti-oxidant effects. In this paper we show that in the presence of Cu(II), curcumin caused breakage of calf thymus and supercoiled plasmid pBR322 DNA. The products were relaxed circles with no detectable linear forms. Other metal ions tested (Mg(II), Ca(II), Fe(II) and Ni(II)) were ineffective or less effective in the DNA breakage reaction. Cu(I) was shown to be an essential intermediate by using the Cu(I)-specific sequestering reagent neocuproine. The involvement of active oxygen species, such as H 2O 2 and 1O 2 was established by the inhibition of DNA breakage by catalase and azide. Curcumin is also able to directly produce O 2 − and H 2O 2 and in the presence of Cu(II), OH · is generated. Absorption spectra of curcumin in the presence of DNA indicated that a complex is formed between the two. The results are discussed in relation to the established pro-oxidant activities of other known anti-oxidants.