Although 10-20 mM MgCl2 in SDS at 37 degrees C extracted significant amounts of Brucella abortus 45/20 peptidoglycan associated proteins (PGAP) from 50 degrees C SDS-extracted cell envelopes, extraction of PGAP of Escherichia coli K-12 required at least 50 mM MgCl2. To study the basis for this difference, PGAP were bound to peptidoglycans and chitin in the presence and absence of lipopolysaccharide and the conditions for their reextraction examined. E. coli PGAP bound to both E. coli and B. abortus peptidoglycans. In either case, 50 mM MgCl2 was necessary for their reextraction regardless of the presence or absence of lipopolysaccharide. E. coli PGAP also bound to chitin and this binding was identical to the binding to peptidoglycans with respect to the temperature of extraction in SDS and, in presence of lipopolysaccharide, the MgCl2 concentration required for reextraction. However, when lipopolysaccharide was absent, 10-20 mM MgCl2 was enough for partial reextraction of PGAP. These results, and our previous finding that B. abortus lipopolysaccharides are not stabilized by interactions with divalent cations, suggested that E. coli lipopolysaccharide remaining in the 50 degrees C SDS-extracted envelopes interfered with the extraction of matrix proteins by MgCl2.