Rat alpha 2-macroglobulin (alpha 2M) is an acute-phase protein synthesized in the liver. Using an in vitro translation system coupled with solid-phase radioimmunoassay, alpha 2M mRNA activity was found to rise to a maximum level in 16-24 h after turpentine injection. Poly(A)+ RNA from turpentine-injected rat liver was converted to cDNA by the method of Okayama-Berg, and about 50,000 transformants were obtained. From these transformants, clones containing alpha 2M cDNA were selected using the following criteria: 1) alpha 2M cDNA should hybridize with synthetic oligonucleotides encoding portions of the alpha 2M amino acid sequence, 2) alpha 2M cDNA should hybridize preferentially with RNA which increases during inflammation, 3) mRNA which hybridizes with alpha 2M cDNA should encode a polypeptide which specifically reacts with antibody against alpha 2M, and 4) the cDNA should contain the nucleotide sequences encoding the amino acid sequences of alpha 2M. We found clones which fulfilled these criteria. Using the cDNA clone as a probe, we demonstrated that the level of alpha 2M mRNA in the liver of inflamed animal markedly increased up to 1000-fold. The size of the alpha 2M mRNA was about 4800 nucleotides in length by Northern analysis.