When cells of a marine pseudomonad (ATCC 19855) were extracted with 45% aqueous phenol, lipopolysaccharide (LPS) could not be recovered from the extract. If the phenol solution used for extraction contained NaCl, KCl, and MgCl2at their concentrations in the growth medium (0.22 M, 0.01 M, and 0.026 Mrespectively) and the fluid used subsequently to dialyze the extract free of phenol was a solution of the same salts, LPS could be sedimented from the aqueous phase of the extract by centrifugation at 144 000 × g. Tests on the individual salts at their growth-medium concentrations showed that the presence of MgCl2alone in the phenol solution gave rise to even higher yields of LPS than did the three salts together. With NaCl in the phenol solution, the yield of LPS was much reduced while with KCl present, either traces or no LPS could be isolated. Qualitative and quantitative analysis of the LPS showed galactose and glucose to be the predominating neutral sugars, and glucosamine and galactosamine the major amino sugars. Two unidentified amino sugars were present in small amounts. They were tentatively identified as a 2-amino sugar with chromatographic characteristics similar to 2-aminoquinovose and a 4-amino sugar. The LPS contained 1.25% heptose sugar, 0.1% 2-keto-3-deoxyoctulosonic acid (KDO), and 12.9% lipid A.