The outer membrane complex of Rochalimaea quintana was isolated by use of ethylene-eiaminetetraacetate and was compared biochemically and biologically both with lipopolysaccharide (LPS) isolated by phenol-water extraction of whole organisms and with lipids isolated by chloroform-methanol extractions of the phenol-water insoluble residues. The outer membrane consisted of protein and LPS components, as distinguished by precipitin tests with sera from patients with trench fever or tests with hyperimmune animal sera. The outer membrane protein component, but not LPS, also reacted with sera from infections with Rickettsia tsutsugamushi. The LPS contained 2-keto-3-deoxy-octonate and heptose. The outer membrane and phenol-extracted LPS were reactive in the chick embryo lethality test, limulus assay, and complement activation. Outer-membrane activity was confined to the LPS component. The lipid extracts were reactive in chick embryo lethality and limulus assays, but did not activate complement.