Rat major acute-phase alpha(1)protein (MAP) was characterized by determining its secondary structure, ligand binding and partial amino acid sequence. A cDNA clone expressing MAP and coding for the entire mature protein was isolated from a cDNA library in E. coli prepared from rat liver mRNA. By hybridization to nick translated cDNA, mRNA for MAP was found only in liver, where it increased 17-fold during acute inflammation. Constant proportions of rates of leucine incorporation into MAP over mRNA levels in liver indicated that the regulation of the synthesis of MAP is due to a change in the rate of synthesis and/or the stability of mRNA for MAP, but not the rate of its translation.