The conditions for conjugated linoleic acid (CLA) production from castor oil, in which the main fatty acid component is ricinoleic acid, were investigated using washed cells of Lactobacillus plantarum JCM 1551 as the catalyst. In the presence of lipase, castor oil became an effective substrate for CLA production by the bacterium. Lipase M "Amano" 10 supported CLA production most effectively among the lipases tested. The addition of a detergent, especially Lubrol PX, enhanced the CLA production. The CLA produced comprised a mixture of two isomers, i.e. cis-9,trans-11-octadecadienoic acid (CLAI) and trans-9,trans-11-octadecadienoic acid (CLA2). Under the optimum conditions (1.0 M sodium citrate buffer, pH 6.0, 37 degreesC) with castor oil as the substrate and washed cells of L. plantarum (12%, wet cell w/v) as the catalyst, 2.7 mg/ml CLA was produced from 5.0 mg/ml castor oil in 99 h (productivity, 0.027 mg/ml/h), and 7.5 mg/ml CLA from 30 mg/ml castor oil in 171 h (productivity, 0.044 mg/ml/h). In the former case, the CLA produced accounted for 45.5% of the total fatty acids obtained, and consisted of CLA1 (26%) and CLA2 (74%). (C) 2004 Elsevier Inc. All rights reserved.