A resting cell biotransformation of phytosterol to androsta-diene-dione (ADD) in cloud point system was carried out. Through optimizations of the resting cell biotransformation conditions, the benefits of biotransformation of hydrophobic compounds in cloud point system were achieved and the ADD concentration reached up to 12,000 mg/l. The advantages of biotransformation using resting cells versus using growing cells, are as follows. The cells can be repeatedly utilized. High cell concentration can be used. Biotransformation can be run under non-sterile condition. Those properties advocate that the protocol can be utilized in large-scale production. (c) 2005 Elsevier Ltd. All rights reserved.