The antibiotics are substances that kill or inhibit the growth of bacteria. Most antibiotics are modified chemically from original compounds found in nature. The antibiotics were frequently used for antimicrobial therapy. The main objective of this study was to determination of the effects of some antibiotics such as ceftazidime pentahydrate, prednisolone, amikacin sulfate, ceftriaxone sodium and teicoplanin on bovine milk lactoperoxidase activity were investigated in this study. Lactoperoxidase was purified using Amberlite-CG-50 resin, CM Sephadex-C-50 ion-exchange and Sephadex-G-100 gel filtration chromatographies from bovine milk. 2,2'-Azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) was used as a chromogenic substrate for spectrophotometric determination of lactoperoxidase activity. Lactoperoxidase was purified 19 fold with a yield of 24.5%. All of the used antibiotics exhibited inhibitory effects on the enzyme and displayed competitive type of inhibition mechanism. IC(50) values of ceftazidime pentahydrate, prednisolone, amikacin sulfate, ceftriaxone sodium and teicoplanin were 0.048, 0.053, 0.26, 0.29 and 1.016 mM, respectively. K(i) constants were calculated as 0.018 +/- 0.0035 mM, 0.019 +/- 0.0005 mM, 0.04 +/- 0.015 mM, 0.10 +/- 0.055 mM and 0.13 +/- 0.022 mM for ceftazidime pentahydrate, prednisolone, amikacin sulfate, ceftriaxone sodium and teicoplanin, respectively. Present study showed that ceflazidime pentahydrate exhibited much higher inhibitory effect at lower concentrations compared to other antibiotics used.