The active thiol ester groups of alpha 2-macroglobulin (alpha 2M) were reacted with a biotin derivative and the sites labelled with avidin-ferritin complexes. Electron micrographs show a strong preference of attachment of the ferritins to the ends of the rods of the H-shaped molecules. A mutual "cross-labelling" was observed in an alpha 2M preparation which yielded dimers of the molecules which must have been formed during purification. The molecules were mostly attached to each other at the ends of the rods of the H-shaped molecules. It is concluded that the thiol esters responsible for the covalent attachment of the proteinases (and other molecules) may be located more in the distal parts of the alpha 2M molecules, while the proteinase molecules are finally trapped near to the centre of the alpha 2M molecules.