The interaction mechanism of prulifloxacin (PL) and calf thymus DNA (ct-DNA) was studied by UV spectra, fluorescence spectra, and hydrodynamic measurements. The binding of ct-DNA and different concentrations of PL was discussed with UV, FL, phosphate effect and ion strength. The denaturation temperature and viscosity were measured. It is obvious that there is a hypochromic effect on the UV spectra with the fluorescence intensity decreasing regularly after addition of DNA and a red shift of the maximum emission peak with a static quenching constant of 3.1 x 10(4) L x mol(-1). The above results show that a binary complex forms between PL and DNA. It is indicated that there is no static-electro interaction between them by phosphate effect. The outer groove binding is suggested by denaturation temperature of DNA increasing (no more than 7 degrees C) and viscosity slightly decreasing.