<jats:title>Abstract</jats:title><jats:p>In the course of our screening program for new bioactive compounds, a naturally new 18-membered macrolide antibiotic,<jats:italic>N</jats:italic>-acetylborrelidin B (<jats:bold>1</jats:bold>) along with borrelidin (<jats:bold>2</jats:bold>) were obtained from the marine<jats:italic>Streptomyces mutabilis</jats:italic>sp. MII. The strain was isolated from a sediment sample collected in the Red Sea at the Hurghada Coast and characterized taxonomically. Additional nine diverse bioactive compounds were reported: 6-prenyl-indole-3-acetonitrile (<jats:bold>3</jats:bold>), sitosteryl-3<jats:italic>β</jats:italic>-<jats:sc>d</jats:sc>-glucoside, campesterol, ferulic acid, linoleic acid methyl ester, linoleic acid,<jats:italic>N</jats:italic>-acetylanthranilic acid, indole 3-acetic acid methyl ester, indole 3-carboxylic acid, and adenosine. Structure<jats:bold>1</jats:bold>was confirmed by in-depth NMR studies and by mass spectra, and comparison with related literature data. The antimicrobial activity of the strain extract and compounds<jats:bold>1</jats:bold>and<jats:bold>2</jats:bold>were studied using a panel of pathogenic microorganisms. The in vitro cytotoxicity of compounds<jats:bold>1</jats:bold>and<jats:bold>2</jats:bold>as well as the crude extract were tested against the human cervix carcinoma cell line (KB-3-1).