Lipoamino acids were found to represent 60% of the total extractable cellular lipids in Flavobacterium meningosepticum and F. indologenes which are known to be opportunistic pathogens. The structures of the lipoamino acids were resolved by chemical and physicochemical methods. Three kinds of lipoamino acids were included in the lipid of Flavobacterium: 3-hydroxyisoheptadecanoic acid amide-linked to serine and esterified to isopentadecanoic acid, and 3-hydroxyisoheptadecanoic acid amide-linked to ornithine and esterified to isopentadecanoic acid or 2-hydroxyisopentadecanoic acid. This type of serine-containing lipid is a novel, rare substance and exhibited high hemagglutinating activity (minimum hemagglutinating concentration was 0.25-0.5 micrograms/ml) with very good, stable dispersion of its liposomes. We called the serine-containing lipid 'flavolipin', based on the genus name of the bacteria. Of the two types of the ornithine-containing lipids, the one that had a nonpolar terminal fatty acid showed higher hemagglutinating activity than the one that had a polar terminal fatty acid. The reconstituted liposomes whose lipid composition was similar to that of the original bacterial cell membrane exhibited definite hemagglutinating activity. As soon as a terminal fatty acid of the lipoamino acids was lost, their biological activity on erythrocytes changed from hemagglutinating to hemolytic, being accompanied by the disappearance of animal species specificity. The mechanism of both the hemagglutination and the hemolysis was discussed.