A serosurvey revealed intense cross-reactivity between Bartonella bacilliformis and Chlamydia psittaci. One of the cross-reacting Bartonella antigens was identified as lipopolysaccharide which reacted with Bartonella as well as with Chlamydia serum antibodies. A monoclonal Bartonella antibody bound to Bartonella lipopolysaccharide as well as to the surfaces of Bartonella bacilliformis and Chlamydia psittaci. It was thus demonstrated that Chlamydia psittaci carries a surface epitope identical to an epitope of Bartonella lipopolysaccharide. The lipopolysaccharide was preliminarily characterized by polyacrylamide gel electrophoresis and by a lectin-binding assay. The lipopolysaccharides of Bartonella bacilliformis and Chlamydia psittaci are not identical.