The serum amyloid A (SAA) proteins make up a multigene family of apolipoproteins associated with high density lipoproteins. They are of ancient origin; the finding of a highly homologous protein in mammals and ducks indicates that SAAs have been in existence for at least 300 million years. The interspecies similarity among the SAAs makes the mouse, in which they have been most thoroughly studied, a reasonable model to use for defining the function(s) of this family of proteins in humans. Originally it was observed that the SAA proteins were made in the liver and represented a set of proteins belonging to acute-phase reactants. SAA3 is a unique member of the SAA multigene family in mice in that its mRNA is also expressed in extrahepatic tissues by a variety of cell types, mainly macrophages and adipocytes. To date, nothing has been reported regarding the fate or function of the SAA3 translation product. To identify the SAA3 protein, we developed SAA3-specific antibodies by immunizing rabbits against a portion of SAA3 protein synthesized in a bacterial fusion protein expression system. Electroimmunoblot analysis of serum and lipoprotein fractions of it showed SAA3 to be associated with high density lipoproteins of mice treated with lipopolysaccharide. Furthermore, a continuous mouse macrophage cell line (J-774.1), when exposed to lipopolysaccharide, expressed SAA3 mRNA in a dose-dependent manner and secreted SAA3 protein. The expression and secretion of SAA3 by macrophages stimulated with lipopolysaccharide suggest a role for this SAA in local responses to injury and inflammation.