Four asymmetric cobalt(III) complexes, Co(bpy) 2 (aip) 3+ , Co(bpy) 2 (pyip) 3+ , Co(phen) 2 (aip) 3+ , and Co(phen) 2 (pyip) 3+ (bpy = 2,2,bipyridine, phen = 1,10-phenathroline), (pyip = 2-(1-pyrenyl)-1H-imidazo4,5-fphen, (aip = 2-(9-anthryl)-1H-imidazo4,5,-fphen, have been synthesized and characterized. Their interaction with calf thymus DNA (CT-DNA) was investigated by physico-chemical methods and photocleavage. The size and shape of the ligands have a marked effect on the DNA-binding affinity of the complexes. Irradiation of pBR322 DNA with these novel cobalt(III) complexes results in nicking of the plasmid DNA. Toxicity and induced cell death investigations revealed that the complexes of pyip had higher toxicity than those of aip.