The effects of gentamicin sulphate, acetyl salicylic acid, ampicillin sodium, paracetamol, potassium penicillin and augmentin were investigated on the in-vitro enzyme activity of catalase. Catalase (CAT:EC220.127.116.11) was purified from bovine liver by a simple and rapid method. The purification process was done by 2',5'-ADP sepharose 4B affinity chromatography. Although the purified enzyme showed a tetrameric band on sodium dodecyl sulphate polyacryilamide gel electrophrosis but bovine liver showed a one band. The enzyme activity was measured spectrophotometrically at 240 nm, according to the method of Aebi. From these six drugs, paracetamol, potassium penicillin and augmentin inhibited the activity of the purified enzyme; gentamicin sulphate, acetyl salicylic acid and ampicillin sodium showed little effect on the enzyme activity. The 15() values for these three drugs were as 4.6, 0.35 and 0.49 mM, respectively. The K-i constants were 20, 25 and 25 mM, respectively and they were competitive inhibitors.