The alpha 2-macroglobulins are large molecular weight proteinase-binding proteins that inhibit the ability of proteinases to hydrolyze protein substrates without suppressing activity against amide or ester substrates. They are also able to protect the active site of bound proteinases from active site inhibitors of suitably high molecular weight. The ability to protect the amidolytic activity of trypsin from the macromolecular active site inhibitor, soybean trypsin inhibitor, was used to demonstrate an alpha 2-macroglobulinlike activity in the blood of the horseshoe crab, Limulus polyphemus and the crustaceans Libinia emarginata (the spider crab) and Cancer borealis (the Jonah crab). The alpha 2-macroglobulinlike activities of L. polyphemus and L. emarginata are sensitive to methylamine, but that of C. borealis is relatively insensitive. The molecular weights (mw) of the trypsin-protecting proteins in L. emarginata and C. borealis, estimated from gelfiltration studies, are, respectively, 480 X 10(3) and 460 X 10(3), and are significantly smaller than that of L. polyphemus (Mr = 570 X 10(3)).