Thirteen new desferrioxamine-type siderophores, designated XI-X6, Etl-Et3, Tel-Te3, and PI, were obtained from cultures of Streptomyces olivaceus TU 27 18 by supplementing the production medium with ornithine or 1,4-diaminobutane, 1,ddiaminohexane, bis( 2-aminoethyl) ether, S-(2-aminoethyl)cysteine, and N-glycylethylenediamine. The stability constants of the ferric complexes of the trihydroxamate ligands, XI-X4, Etl-Et3, and Tel-Te3, were determined by EDTA competition reactions and are in the range log pll0 = 29.7-31.8 = [FeL]/([Fe3+][L3-], 25 OC, 0.1 M KN03). The stability of the complexes decreases monotonically as the structure of the ligand differs from that of desferrioxamine E = 32.21, 25 "C, 0.1 M KNO,), the main siderophore of S. olivaceus TU 2718 under natural growth conditions. The dihydroxamic acid desferrioxamine X5 forms two types of ferric complexes, depending on pH. The predominant species above pH 6.4 is a dimer of formulation Fe2L3, which dissociates into a monomeric cation, FeL', at lower pH. The stability constants of these two species determined by spectrophotometric titration are log &30 = 55.35 and log = 19.18 (25 OC, 0.1 M KNO,). The relative effectiveness of the new ferrioxamines as mediators of iron uptake via the hydroxamate ferric ion transport system of Staphylococcus aureus DSM 799 differs widely, however, without correlating with the complex stability.