<p>The potential antibacterial activity of secondary metabolites produced by <i>Streptomyces</i> sp. 156A isolated from Lake Baikal was investigated. The selective liquid–liquid extraction method was applied to obtain a mixture of nactins (polynactin) produced by the strain. The polynactin consisted of nonactin (3%), monactin (18%), dinactin (36%), trinactin (31%) and tetranactin (12%). The compounds were identified by MS/MS, <sup>1</sup>H and <sup>13</sup>C NMR methods. The loss of neutral 184 and 198 Da fragments from a sodiated molecular ion, [M + Na]<sup>+</sup>, of nactins was observed in the MS/MS spectrum. The polynactin was shown to possess the antibiotic activity against Gram-positive strains including opportunistic strains and strains isolated from various ecosystems of Lake Baikal.</p
