The producing strain Streptomyces sp. neau-D50 was isolated by moist incubation and desiccation method from healthy soybean root in Harbin, Heilongjiang Province, China. Streptomyces sp. neau-D50 was maintained on the yeast extract-malt extract-soluble starch (YMS) medium containing soluble amylum (10 g), yeast extract (2 g), KNO₃ (1 g) and agar (20 g) in 1 L tap water, pH 7. The seed medium consisted of glucose (20 g), soybean flour (15 g) and yeast autolysate (5.0 g) in 1.L water, pH 7. All the media were sterilised at 121℃ for 20 min. A slant culture was incubated for 6-8 days at 28℃ . A total of 10 mL of sterile water was added tothe slant of the YMS medium. The spores were scraped off and transferred into a sterile tube containing glassbeads, and the resulting spore suspension was then filtered through six layers of a sterile cheesecloth and adjusted to 10⁷ -10⁸ colony-forming units (c.f.u.) m/L using YMS medium. A 250-mL flask containing 20 mL of seed medium was inoculated with the spore suspension (2 mL) and incubated at 28℃ for 24 h, on a shaker operating at 250 r.p.m. Then, 8 mL of the culture was transferred into a 1-L Erlenmeyer flask containing 100 mL of the fermentation medium comprising glucose (0.5%), lactose (1.5%), cotton seed powder (2.0%), FeSO₄.7H₂O (0.6%), MnSO₄.H₂O (0.2%), MgSO₄.7H₂O (0.2%) and CaCO₃ (0.3%), pH 7. Fermentation was carried out at 28℃ for 7 days on a rotary shaker at 250 r.p.m.