Abstract: The intercalative binding interaction of two naphthyl imides, a N(N,Ndimethyl) ethylamine1,8naphthyl thiaimide) and b (N(N,Ndimethyl) ethylamine1,8naphthalimide) with doublestranded calf thymus DNA has been studied, and an intrinsic binding constant has been obtained. Complexes have been found to promote cleavage of plasmid pUC19 DNA from the supercoiled form I to the open circular form II upon irradiation. The mechanism involved in the photocleavage of the plasmid by the complexes has also been established.