in this study we determined some properties of the cholesterol oxidase from a brevibacterium strain isolated from buffalo's milk and identified the cholesterol degradation products by the bacterial cell. a small fraction of the enzyme synthesized by cells cultured in liquid medium for 7days was released into the medium whereas a larger fraction remained bound to the cell membrane. the extraction of this fraction was efficiently accomplished in 1 mm phosphate buffer, ph 7.0, containing 0.7% triton x-100. the enzyme stability under freezing and at 45oc was improved by addition of 20% glycerol. the optimum temperature and ph for the enzyme activity were 53°c and 7.5, respectively. the only steroidal product from cholesterol oxidation by the microbial cell and by the crude extract of the membrane-bound enzyme was 4-colesten-3-one. chromatographic analysis showed that minor no steroidal compounds as well as 4-colesten-3-one found in the reaction media arose during fermentation process and were extracted together with the enzyme in the buffer solution. cholesterol oxidation by the membrane-bound enzyme was a first order reaction type.