Production of protease by a local wild-type strain, Aspergillus terreus IMI 282743 using pre-filtered palm oil mill effluent (POME) and its derivatives was investigated. POME was first subjected to depth and surface filtration to remove the coarse solids found in the wastewater. Then, the pre-filtered POME was concentrated twofold to form retentate by means of ultratfiltration process. Maximum protease activity of 129 U/ml was achieved on the fourth day of fermentation in the medium containing 75% (v/v) retentate of pre-filtered POME as the sole carbon and nitrogen sources, operated at 37 degrees C, 250 rpm and 5% (v/v) of inoculum concentration. The results indicated that addition of nutrients and adjustment of initial pH were dispensable in this particular study to obtain better yield of protease. On the other hand, a decrease of about 59% and 19% of maximum protease activity found in the medium of pure retentate (100%, v/v) compared to-the medium of 75% (v/v) and 50% (v/v) retentate, respectively, suggesting that retentate should be diluted prior to the fermentation process in order to gain better yield of protease. (c) 2006 Elsevier Inc. All rights reserved.