The ornithine-containing lipids of six strains (phases I-IV) of Bordetella pertussis were prepared from the total extractable cellular lipids by thin-layer chromatography and treatment with phospholipase A. They were compared with those prepared from two strains each of Bordetella parapertussis and Bordetella bronchiseptica. The structures of the ornithine-containing lipid of B. pertussis and the other two species were resolved by acid and alkaline hydrolysis, gas-liquid chromatography, infrared absorption spectroscopy, amino acid analysis and combined gas-liquid chromatography/mass spectrometry. The main structure of the aminolipid of the three species of Bordetella was 3-hydroxyhexadecanoic acid, amide-linked to ornithine and esterified to the second hexadecanoic acid. The aminolipid of B. pertussis Sakurayashiki (phase III) exhibited high hemagglutinating activity for human and rabbit erythrocytes, having a minimum hemagglutinating concentration of 1 microgram/ml against 8-16 micrograms/ml for the other strains of Bordetella. All of these aminolipids showed some degree of microheterogeneity. Because the 3-hydroxyhexadecanoic acid content was especially high in strain Sakurayashiki, it was presumed that the intensity of hemagglutinating activity of the aminolipid was affected by the chain length of the central 3-hydroxy fatty acid, that is the aminolipid containing 3-hydroxyhexadecanoic acid had high hemagglutinating activity. The hemagglutination was inhibited by phosphatidylcholine at concentrations of more than 20 micrograms/ml. Other inhibitory substances were cysteine, sphingomyelin, acidic amino acids, histidine, unsaturated fatty acid and basic amino acids. Furthermore, the divalent cations Ca2+ and Mg2+ inhibited this hemagglutination at a concentration of 1 mM. The O-deacylated ornithine-containing lipid that had lost hexadecanoic acid did not have any hemagglutinating activity but did have hemolytic activity. Observation by electron microscopy indicated that erythrocytes were combined by the liposomes of the ornithine-containing lipids. On the basis of these results, the proposed mechanism of hemagglutination by the aminolipids is that the liposomes of the aminolipids combine erythrocytes by hydrophobic interaction between the fatty acid moieties of the aminolipid and the lipids of the surface of erythrocytes, and by ionic interaction between the ornithine of the aminolipid and the protein of the surface of the erythrocytes. In addition, the hemagglutinating activity of phosphatidylserine was found to be due to its similar structure to that of the ornithine-containing lipid and the mechanism was also presumed to be similar. The mechanism of hemagglutination by these aminolipids was distinct from that of lectins.