The three-dimensional reconstructions of the human plasmin alpha(2)-macroglobulin binary complex were computed from electron microscopy images of stain and frozen-hydrated specimens. The structures show excellent agreement and reveal a molecule with approximate dimensions of 170 (length) x 140 (width) x 140 Angstrom (depth). The asymmetric plasmin structure imparts significant asymmetry to the plasmin alpha(2)-macroglobulin complex not seen in the structures resulting from the reaction of alpha(2)-macroglobulin with methylamine or chymotrypsin. The structure shows, when combined with other studies, that the C-terminal catalytic domain of the rodshaped plasmin molecule is entrapped inside of the alpha(2)-macroglobulin cavity, whereas its N-terminal kringle domains protrude outside one end between the two arm-like features of the transformed alpha(2)-macroglobulin structure. This arrangement ensures that the catalytic site of plasmin is prevented from degrading plasma proteins. The internalized C-terminal portion of the plasmin structure resides primarily on the major axis of alpha(2)-macroglobulin, suggesting that after the initial cleavage of the two bait domains and the thiol esters, the rod-shaped plasmin molecule enters the alpha(2)-macroglobulin cavity through the large openings afforded by the half-transformed structure. This mode of entrapment requires the untwisting and the separation of the two strands that constitute the alpha(2)-macroglobulin structure.