Received 26 January 1998; received in revised form 19 May 1998; accepted 20 May 1998 The results from the measurement of the fluorescence spectrum showing the binding characteristics of acridine orange (AO) and bovine serum albumin (BSA) are reported. It was found that the equilibrium constant k-4848.64 lmol1, and the number of binding sites n-0.82. Based on the mechanism of the Forster energy transference, the transfer efficiency of energy and distance between the acceptor AO and BSA were found. The interaction between AO and BSA have been verified as consistent with the static quenching procedure and the quenching mechanism is related to the energy transfer.