We qualitatively and quantitatively analyzed the simple and complex lipids of 10 Legionnaires' disease bacteria. The phospholipids in decreasing order of concentration were phosphatidylcholine, phosphatidylethanolamine, cardiolipin, phosphatidylmonomethylethanolamine, phosphatidylglycerol, and phosphatidyldimethylethanolamine. The total phospholipids averaged 96 micromoles per gram dry cell weight. Phospholipid fatty acids were solely branched-chain fatty acids and were, in decreasing order of concentration, iso-C16:0, anteiso-C15:0, anteiso-C17:0, iso-C14:0, iso-C16:l, and an unidentified fatty acid. Neutral lipids identified were free fatty acid, ubiquinone, triglyceride, diglyceride, monoglyceride, and wax ester. Neutral lipid fatty acids consisted predominately of branched-chain fatty acids, normal fatty acids, and a minor unidentified fatty acid. Analysis of the cellular lipids of 10 Legionnaires' disease bacteria revealed an unusual and novel microorganism.