Mercury(II) porphryin enhanced DNA cleavage in the presence of 0.2 units muL(-1) of HaeIII at which concentration the restriction enzyme could not cleave DNA in the absence of the porphyrin. This was ascribed to the synergistic effect of the bound Hg2+ ions to DNA and the intercalated free base porphyrin, released from the mercury(II) porphyrin complex upon binding to DNA, which was confirmed by UV-visible and CD spectroscopic measurements.