When Salmonella minnesota R595 lipopolysaccharide (LPS) is mixed with serum, the LPS eventually forms a complex with high density lipoprotein (HDL). Complex formation is conveniently followed by CsCl equilibrium density gradient centrifugation. When mixing 10 micrograms LPS with normal rabbit serum (NRS) at 37 degrees C in the presence of 20 mM EDTA, the half-life for LPS binding to HDL is typically 2 to 3 min. When the same experiment is performed with the use of acute phase rabbit serum (APRS; collected 24 hr post-induction with silver nitrate), the half-life for LPS binding to HDL is typically 40 to 100 min. Thus LPS binding to HDL occurs some 20- to 40-fold slower in APRS than in NRS. Two other phenomena have been found, the time dependencies of which correlate well with the time dependency of LPS binding to HDL in APRS. If LPS-APRS reaction mixtures are cooled to 4 degrees C shortly after mixing and are dialyzed against 2.5 mM HEPES, 15 mM NaCl, pH 7.4 buffer, LPS is recovered in the washed precipitates ("euglobulin precipitate") if, and only if, the LPS-HDL binding reaction is not complete. The amount of LPS in the precipitate correlates well with the amount of LPS that has not bound to HDL. The second phenomenon we observe is that the LPS-containing euglobulin precipitate prepared from LPS-acute phase serum reaction mixtures shortly after mixing also contains a protein, gp60, the concentration of which in the euglobulin precipitate correlates well with the amount of LPS in the precipitate. Thus three phenomena are kinetically well correlated in APRS: the degree of binding of LPS to HDL, the degree of appearance of LPS in a euglobulin fraction, and the concentration of protein gp60 in the euglobulin fraction. We were unable to precipitate gp60 from APRS in the absence of LPS, from APRS after the LPS has fully bound to HDL, or from normal serum in the presence or absence of LPS. The known properties of gp60 are not reminiscent of any other known acute phase reactant. These data demonstrate that APRS contains acute phase reactants that interact with LPS to modify its buoyant density, its solubility, and the rate of its binding to HDL.