The chemical and biological characteristics of Soluble Protective Antigen (SPA) separated from culture fluids of Salmonella enteritidis strain 2547 were analysed. It was shown that SPA has 3-hydroxy, nonpolar fatty acids by thin-layer chromatography. The fatty acids were identified as lauric, myristic, palmitic and 3-hydroxymyristic acid using gas chromatography-mass spectrometry and mass chromatography. These fatty acids are common constituents of the lipid A obtained from S. enteritidis. SPA was found to enhance the plaque-forming cell (PFC)-response to sheep erythrocytes in the recipients' spleen. In addition, SPA enhanced the clotting activity of Limulus amebocyte lysate. These results show that SPA possesses the properties of lipopolysaccharides isolated from strain 2547 by chemical procedures.