Twenty strains of filamentous fungi (from nine genera) and four species of bacteria were screened for their capabilities to transform bufadienolides. The highly stereo- and regiospecific isomerization and dehydrogenation of three major bufadienolides (resibufogenin, cinobufagin and bufalin) at 3-OH by Fusarium solani AS 3.1829 were reported. Three major transformed products were obtained and their structures were identified as 3-epi-resibufogenin (1a), 3-epi-bufalin (2a), 3-keto-cinobufagin (3a), respectively, on the basis of their spectral data. The reaction kinetics and the effects of pH, substrate concentration and incubation phase on the biotransformation were also investigated. (C) 2006 Published by Elsevier Inc.