The fatty acid composition of the total extractable cellular lipids of Bordetella pertussis was very characteristic and was mostly hexadecenoic and hexadecanoic acids (90%) in a ratio of about 1:1. The fatty acid composition of Bordetella parapertussis and Bordetella bronchiseptica differed from that of B. pertussis. The two species were distinguished by the fatty acid composition of cell-bound lipids. The ornithine-containing lipid was characteristic of the genus Bordetella and its main structure was 3-hydroxyhexadecanoic acid amide-linked to ornithine and esterified to the second hexadecanoic acid. The lipid agglutinated human and some animal erythrocytes. The lipid is a new type of hemagglutinin and we proposed that hemagglutination occurred mainly by the hydrophobic interaction between the lipid moiety of the ornithine-containing lipid and phosphatidylcholine in the cell membrane of the erythrocytes. A relatively high content of ornithine-containing lipid was also found in opportunistic pathogens such as Flavobacterium meningosepticum which causes meningitis in babies and children. As the pathogenicity of the opportunistic pathogens is unclear, the ornithine-containing lipid may have an important role in pathogenicity.