A model of alpha 2-macroglobulin is presented that is compatible with previous structural, functional, and phylogenetic studies of the protein. The model of the molecule resembles a hollow cylinder and is comprised of two identical functional halves with three C2 axes of symmetry and no mirror planes. The "trap mechanism" of this proteinase inhibitor is effected by slight movement of two trap arms per "half-molecule." Evidence for this model is obtained from the study of the structure and proteinase binding of the molecule. By using this model, predictions are made concerning proteinase binding ratios, receptor recognition, and "slow-to-fast" conformational change of the molecule.