The antibiotic actinonin, isolated from strains of Streptomyces (including Streptomyces sp. Cutter C12 and Streptomyces roseopallidus DS 40562), was found to have the molecular formula C₁₉H₃₅N₃O₆. Its novel pseudopeptide structure (XII) was elucidated through chemical analyses—including vigorous/mild acidic hydrolysis (yielding D-pentylsuccinic acid, L-valine, L-prolinol, and hydroxylamine) and sodium hydrogen carbonate hydrolysis—and spectral studies (IR, UV, NMR, mass spectrometry), as well as Lossen degradation (confirming the sequence via isocyanate formation and hydrolysis to 2-(aminomethyl)heptanoic acid). Key structural features include: being the first known naturally occurring L-prolinol derivative; representing the first natural hydroxamic acid of the RCO·NH·OH type; containing only L-amino acid residues, with the pentyl side chain of D-pentylsuccinic acid being isosteric to L-amino acid side chains (justifying its pseudopeptide classification). The mode of biological action of actinonin is proposed to involve ribonucleic acid synthesis, and structural-activity relationships note retained activity in synthetic analogs where L-proline is replaced by L-prolinol.