The in vitro DNA interactions and bacterial mutagenicities of cis-[CrIII(phen)2(OH2)2] 3+ and trans-[CrIII(salen)(OH2)2]+ and their CrV analogues are reported. At pH 3.3, cis-[Cr(phen)2(OH2)2]3+ (0.02-2.0 mM) causes negatively supercoiled pUC9 DNA to smear on agarose gels, with substantial precipitation in the well at ≥1.0 mM. Much weaker interactions between CrIII and DNA were apparent at pH 7.4. The interactions between DNA and CrV phen complexes (0.5 mM total Cr, pH 3.3) generated by oxidation of cis-[Cr(phen)2(OH2)2]3+ (for 10-30 min) resulted in almost complete nicking of form I DNA to forms II and III DNA. Nicking of form I DNA (≥80%) was also apparent at pH 7.4 following reaction of DNA with PbO2-oxidized [Cr(phen)2(OH2)2]3+ (2 mM Cr). Interactions between trans-[CrIII(salen)(OH2)2]+ and DNA were weaker than those of the CrIII phen complex at both pH 3.3 and 7.4. The CrV salen derivative (0.5 mM total Cr) caused the disappearance of form I DNA at oxidation times of ≥10 min and at pH 3.3 with substantial cleavage. While oxidation of [Cr(salen)(OH2)2]+ by PbO2 was not observed at pH 7.4, the complex was oxidized by iodosobenzene to produce short-lived [CrO(salen)]+ that caused DNA smearing on the agarose gel. In bacterial mutagenicity assays, the CrIII imine complexes and their CrV analogues produced similar mutagenic responses, which were believed to be due to the instabilities of the CrV species in the bacterial growth medium. While the spectrum of the mutagenic activities differed between the chromium phen and salen complexes, both exhibited greatest mutagenicity in Salmonella typhimurium TA102. These data suggest that CrV species, generated in vivo by cellular oxidative enzymes, may be responsible for CrIII-induced mutagenesis. © CSIRO 2000.