Biotransformation of puerarin catalyzed by filamentous fungi has been studied. A primary screening was carried out using 138 filamentous fungi isolated from soil, that was able to grow in the presence of puerarin. Strain NJ01 was identified as Trichoderma harzianum. Under the optimum conditions, resting mycelium of T harzianum NJ01 converted 41% of added 0.6g/l puerarin to 3'-hydroxypuerarin that was characterized by MS, C-13 NMR and H-1 NMR spectra. In the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical-scavenging system, the activity of 3'-hydroxypuerarin (3'-OHP) was 20 times higher than that of puerarin. The solubility of 3'-OHP was 1.3 times greater than that of puerarin. (c) 2006 Elsevier Inc. All rights reserved.