In a recent communication we reported the isolation and structure elucidation of leucinostatin D, a minor, biologically active, peptide component of the antibiotic mixture produced by submerged cultures of Paecilomyces marquandii (Massee) Hughes. In a search for further biologically active metabolites, the darkbrownish oily residue from the benzene extract of the culture broth was subjected to repeated flash chromatography on silica gel columns using CHCl3 as the eluent to which MeOH and NH3 were added in continuously increasing proportions up to final composition CHCl3-MeOH-NH3, 80:18:2. At the end of this procedure, a fraction was obtained that consisted mainly of two new peptidic components we have labeled leucinostatins H and K. These were separated and purified through repeated flash chromatography under N2 atmosphere using final solvent mixture CHCl3-MeOH-NH3 as the eluent. Evaporation of the solvents gave leucinostatin H (1, Rf 0.4) as a white, partly crystalline material (mp 166~168°C) and leucinostatin K (2, Rf 0.45) as white amorphous solid (mp 138~141°C). While the fourier transform (FT) IR and UV/vis spectra of the new metabolites provided values very similar to those obtained for other leucinostatin components described earlier, fast atom bombardment (FAB) mass spectra, remarkably displaying the M+H ions only, suggested that peptides 1 [m/z (nominal mass) 1,134] and 2 (m/z 1,234) might correspond to leucinostatins D (3, m/z 1,118) and A (4, m/z 1,218) with an additional oxygen atom in the molecules. Supporting evidence for this structural relationship and information as to the site of oxidation were conveniently inferred from the highfield NMR spectra. Detailed analysis of the conventional (1D) and two-dimensional (2D) 13C (100MHz) and 1H (400MHz) spectra disclosed that 1 and 3 and, in a similar manner, 2 and 4 consist, pairwise, of the same amino acid residues arranged in identical sequences. Multiplicity-selected carbon-13 spectra revealed that incorporation of the extra oxygen atom into the metabolites entails no changes in the total number of carbon-bonded hydrogen atoms, a finding that suggests N-oxidation. From the comparison of the fully assigned 13C and 1H chemical shift data of 1 and 2 with those of their assumed non-oxygenated counterparts, 3 and 4, it became evident that the oxidation-induced changes in the chemical shifts are restricted to nuclei in the 1-(dimethylamino)-2-aminopropane unit, indicating oxygen substitution at the quaternary nitrogen atom. Corroborative chemical evidence for the correctness of structures 1 and 2 was readily available by N-oxidation of 3 via m-Cl-perbenzoic acid treatment (in dry CH2Cl2 at room temperature), resulting in a product identical with 1. Leucinostatins H and K show biological activity against Gram-positive bacteria and fungi as do the respective leucinostatins D and A, but both the antibacterial and the antimycotic activities become significantly reduced upon N-oxidation. N-oxidation also causes a nearly 2-fold decrease in the phytotoxic activity; irreversible withering of tomato cuttings became visible after 72 hours at 10 and 5 μg/ml concentrations of 1 and 2, respectively. N-Oxides are known to occur frequently in nature and are generally believed to be connected with processes leading to N-dealkylation of natural products. Sizable amounts of nor- and bisnor-leucinostatin A have been detected in the culture filtrates of P. marquandii. A recent communication by Stroh et al. reported the detection of two minor components from Paecilomyces lilacinus A-257 with mass values suggesting they may be identical with leucinostatins H and K.