Abstract: The binding reaction between bovine serum albumin (BSA) and large ring lactone antibiotics,taking roxithromycin (RM) and azithromycin (AM) as example,was studied by fluorospectrophotometry and UV-VIS spectrophotometry.As shown by the experimental results,either of the antibiotics were bound strongly to BSA to form compound complex with mole ratio of 1 to 1 and equilibrium constants of KRM=3.6×104L·mol-1 and KAM=7.2×104L·mol-1.The action distance,r between the donor (BSA) and the acceptors (RM,AM) and their efficiency of energy transfer (E) were found,according to the principle of Frster′s non-nonradiative energy transfer,to have the following values: rRM=0.49 nm,rAM=1.58 nm,ERM=0.738,EAM=0.779.As proved by the Stern-Volmer equation,the fluorescence quenching of BSA caused by RM (or AM) was a static quenching process due to the binding reaction between BSA and RM (or AM).