Numerous papers have been published on bacterial polysaccharides containing amino sugars or aminouronic acids, and on oligosaccharides containing amino sugars in glycoproteins, glycosaminoglycans and glycolipids. The title compound (2-amino-2-deoxy-D-mannitol, 3) was previously reported as its acetate only for the purpose of analyzing the 2-aminoaldohexoses of bacterial heteroglycans, but no work has been reported on its microbial origin. Since we reported the production of moranoline (1) by Streptomyces sp., we have been studying a more advanced fermentation to improve the productivity of 1 for industrial purposes, and observed a few by-products which interfered with the purification of 1; one of these by-products was revealed to be 1,5-dideoxy-1,5-imino-D-mannitol (2). In the present paper, we describe the isolation of another by-product (3) from the culture broth of Streptomyces lavendulae GC-148, a producing strain of 1 and 2. The strain was cultivated on a rotary shaker and in a jar fermentor with a medium containing soluble starch, soybean meal, yeast extract, NaCl, NaNO3, KCl and MgSO4·7H2O (pH 7.2). The broth filtrate was applied to a Dowex 50Wx2 (H+) column, desorbed with 1 N aqueous ammonia, then to a Diaion SAllA (OH-) column. The physico-chemical properties of the compound were determined: mp 161-163°C, [α]D24 +4.0° (c 1.0, H2O), anal. Calcd. for C6H15NO5: C 39.77, H 8.34, N 7.73; Found: C 39.62, H 8.17, N 7.81, mass spectrum peaks at m/z 182 (M++1, 4.1%) and 183 (M++2, 0.5%), 1H NMR (in D2O) δ 2.98-3.12 (1H, m) and 3.56-3.90 (7H, m), 13C NMR (in D2O) δ 53.84 (CH), 64.05 (CH2), 64.31 (CH2), 70.73 (CH), 71.29 (CH), 71.85 (CH). The compound was identified as 2-amino-2-deoxy-D-mannitol (3) by comparing with synthetic 2-amino-2-deoxy-D-mannitol, which was prepared via sodium borohydride reduction and consecutive N-deacetylation of N-acetyl-D-mannosamine. Furthermore, supplementation of 3 at levels of 0.5% and 1% to the culture medium of S. lavendulae NL-476 (a natural isolate of GC-148) increased the yield of 1 from 3601 µg/ml to 4969 µg/ml and 6181 µg/ml, respectively, suggesting that 3 is closely involved in the biosynthesis of 1.