Twenty-four oligoamines belonging to six (1-6) structurally different types were tested in vitro for their antibacterial activity against 14 different bacterial species comprising a total of 187 strains. Ten compounds were able to inhibit growth of at least one strain at concentrations < or = 10 mumol/L. For three compounds, minimum inhibitory concentrations for some strains were even below 1 mumol/L. Clear structure-activity relationships showed that the inhibitory effect depended on the bridge connecting the nitrogen atoms, the substitution of the nitrogens, and the number of nitrogen atoms present in one molecule. Substitutions like N-4-phenylbutyl, N-octyl, and N-nonyl were most active, while short (butyl) and long (dodecyl) substituents diminished or abolished the activity. The antimicrobial spectrum of the oligoamines tested here covered gram-positive (e.g. Staphylococcus aureus, Listeria monocytogenes, Bacillus subtilis) and gram-negative (e.g. Escherichia coli, Citrobacter spp., Acinetobacter spp.) microorganisms. The type of action was classified as bactericidal. As the inhibition of growth is complete immediately after the addition of the oligoamines, an interaction with the bacterial cell-membrane is probable.