A high molecular weight protease inhibitor has been purified from the cell-free plasma of the horseshoe crab Limulus polyphemus using high speed centrifugation, polyethylene glycol precipitation, and gel filtration. The inhibitor is sensitive to mild acidification, methylamine treatment, and inhibits the proteolytic activity of a variety of endopeptidases. The molecule does not inhibit trypsin-mediated hydrolysis of low molecular weight substrates and protects the active site of trypsin from inactivation by soybean trypsin inhibitor. These properties are diagnostic of the alpha 2-macroglobulin (alpha 2M) class of protease inhibitors found in vertebrates. Like vertebrate alpha 2M the Limulus alpha 2M molecule is composed of subunits of molecular weight 180,000-185,000 as determined by polyacrylamide gel electrophoresis under reducing conditions. The apparent native molecular weight for the Limulus molecule as determined by both gel filtration and gel electrophoresis under nonreducing conditions is 500,000-550,000, compared to a native molecular weight of 700,000-750,000 for human alpha 2M, determined in parallel under identical conditions. These results suggest that alpha 2M appeared in evolution at least 550 million years ago before the divergence of the lineages that gave rise to present-day arthropods and mammals.