The synthesis of complex lipids by Borrelia hermsi while growing in Kelly medium was investigated by labeling cultures with D-[14C]glucose or [14C]palmitic acid. Labeled glucose was incorporated into phosphatidyl choline, phosphatidyl glycerol, monogalactosyl diglyceride, cholesterol glucoside, and acylated cholesteryl glucoside. The fatty acid composition reflected that of the medium suggesting that this spirochete directly incorporates acyl chains unaltered from its external milieu. Furthermore, the distribution of labeled carbon between acyl groups (lipid-soluble) and water-soluble moieties indicates that there is no metabolic exchange between these two pools. The relationship of B. hermsi to other spirochetes is discussed in terms of lipid metabolism as a generic characteristic.