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The effects of pH, chloride ion, and fatty acids on the interaction between methacycline (MTC) and human serum albumin (HSA) were fluorometrically examined. The fluorescence intensity and binding constant (K) of MTC-HSA complex increased with pH from 6.4 to 8.4,suggesting that MTC binding to HSA is significantly affected by conformational change of HSA in this pH range. Chloride ion effectively displaced MTC from its binding site when HSA was in the basic conformation. The binding affinity of MTC for fatty acid-free HSA depended upon the concentration of fatty acids (oleic and palmitic acids) at physiological pH. Specific binding sites for MTC on HSA were also examined in connection with Sudlow's classification. It was found that the primary binding site of MTC on HSA is site 1.