The chemical composition of the outer envelope (OE) of Treponema phagedenis biovar Kazan 5 was investigated. After cultivation in a lipid-defined medium, the OE was removed from the cells with 0.7 mM sodium dodecyl sulfate. The solubilized OE was reaggregated by dialysis against 20 mM MgCl2, washed, lyophilized, and subjected to chemical analysis. The average yield of OE was 14.6% of the whole cell (WC) dry weight. The magnesium content was 0.683 mug/mg OE. Peptidoglycan components such as muramic acid and ornithine were detected in the WC but not in the OE, and diaminopimelic acid was absent in both WC and OE. The OE contained protein (60-73%), carbohydrate (1-2%), and lipid (4-5%), primarily polar lipid. The major polar lipids were monogalactosyldiglyceride (43%) and phospholipid (57%), of which phosphatidylcholine was the main phospholipid component, with phosphatidylethanolamine present in lesser amounts.