Patients with systemic lupus erythematosus were examined for the presence of serum antibodies reactive with phospholipids (PL) extracted from the membranes of Klebsiella K30 (K30PL). Affinity-purified anti-K30PL antibodies were tested for their ability to cross-react with other PL antigens or DNA and for the presence of idiotypic markers known to be associated with anti-DNA antibodies or antibodies to Klebsiella K30 polysaccharide (K30p). Affinity-purified antibodies to K30PL, phosphatidylserine (PS), or phosphatidylinositol (PL) uniformly cross-reacted with each other. Analysis of the PL preparations by thin-layer chromatography and reversed-phase high-performance liquid chromatography (HPLC) showed the presence of several components. Lupus sera reacted with one component mainly in the HPLC-fractionated K30PL preparation, although this component appeared to be present in the other PL preparations. Direct-binding and inhibition studies showed that affinity-purified antibodies to the K30PL extract, PS, or PI reacted poorly with DNA. However, the anti-K30PL antibodies possessed a prominent anti-DNA idiotypic marker (AM Id) in 61% of the patients and an anti-K30p idiotypic marker (SP Id) in 94% of the patients. The results thus show that anti-K30PL antibodies are idiotypically related to anti-DNA and anti-K30p antibody subpopulations, although they do not share the same antigen-binding characteristics.