The microbial transformation of podophyllotoxin by endophytic actinomycetes isolated from the rhizoma of Taoerqi (Sinopodophyllum hexandrum) was investigated to obtain some structural analogs or derivatives of podophyllotoxin. Endophytic actinomycetes were isolated from the rhizoma of S. hexandrum by surface-sterilization method, and screening test for the microbial transformation of podophyllotoxin was carried out using thin layer chromatog. and high performance liquid chromatog. (HPLC). Bio-transformed products of podophyllotoxin were isolated and purified by silica column chromatog. and preparative HPLC, and chem. structures of transformed products were elucidated on the basis of chem. spectral data. The endophytic actinomycetes were identified by morphol. characteristics, physiol. and biochem. characteristics, and the anal. of fragment of 16S rRNA gene. The results showed that among totally isolated 20 endophytic actinomycetes, one was found to be able transform podophyllotoxin into 4'-demethylepipodophyllotoxin, which was identified preliminarily as Streptomyces sp. Endophytic actinomycetes Streptomyces sp. was able to modify the chem. structure of podophyllotoxin by demethylation and isomerization reaction, it was deduced that this actinomycetes possessed O-demethylation and isomerization enzyme.