Interest in the interaction between cationic porphyrins,particularly derivatives of meso-tetra(N-methylpyridinium-4-yl)porphyrin(TMPyP),and DNA abounds because they are versatile DNA-binding agents that could find application in photodynamic therapy,cancer detection,artificial nucleases,virus inhibition and so on.The interaction of two water-soluble cationic porphyrins,meso-tetrakis(4-N,N,N-trimethylanilinium)porphyrin (TMAP) and 5-phenyL^-10,15,20-tris[4-(N-methyl)pyridinium]porphyrin (TriMPyP),with calf thymus DNA (ctDNA) was studied by UV-Vis absorption spectroscopy,fluorescence spectroscopy and resonance light scattering technique.TriMPyP forms aggregate in water due to the molecular asymmetry while TMAP exists as monomers.At lower concentrations of ctDNA (R>1,R =cTMAP/cDNA base pair),the interaction of TMAP with DNA leads to significant hypochromicity and bathochromic shift of absorption spectra.And the fluorescence of TMAP was quenched while it showed enhanced resonance light scattering signals.But the extent of enhancement of resonance light scattering signals is very small,so the aggregate of TMAP is not very high.These observations indicate the self-stacking of TMAP along the DNA surface.At higher concentrations of ctDNA (R<1),TMAP association with DNA is via outside binding which is accompanied with hyperchromic effect and fluorescence enhancement while the resonance light scattering signals is reduced.DNA addition decreases the fluorescence intensity of TriMPyP and it shifts the peak to the higher wavelengths (red shift).The interaction with DNA promotes the aggregation of TriMPyP and no simple outside binding is observed even at higher concentrations of ctDNA.The steric effect of molecular distortion constrains the intercalation or further binding to DNA.The effect of ionic strength on the interaction was investigated at two DNA concentrations,1.2 and 24.0 μmol·L-1,for TMAP.The Interactions of both porphyrins with DNA show high sensitivity to ionic strength.By addition of NaCl,electrostatic attraction is decreased,resulting in the change of binding mode.