Crystals of methylamine‐treated α2‐macroglobulin (α2M‐MA), α2‐macroglobulin in complex with two molecules of trypsin, α2M‐T2, one molecule of plasmin, α2‐M‐PL, and one molecule of plasmin followed by methylamine‐treatment, α2M‐PL(MA), have reproducibly been obtained using ammonium sulfate or magnesium sulfate as precipitants. The crystals are fragile tetragonal bipyramids of up to 1.5 mm in length. Crystals of α2M‐MA diffracted to at least 9A˚resolution, crystals of α2M‐T2 diffracted to 10A˚resolution and crystals of α2M‐PL and α2M‐PL(MA) diffracted to 11A˚resolution. For α2M‐MA the cell parameters were determined as:a=b=257A˚,c=555A˚; and for α2M‐T2 as:a=b=247A˚,c=559A˚. For both preparations the space group was I4(1)22. As estimated from density measurements, the crystals of α2M‐MA and α2M‐T2 contain one 360 kDa α2M dimer per asymmetric unit. The volume of the asymmetric unit/molecular weight,V m, was estimated at 5.6A˚3/Da. The crystal parameters of α2M‐PL and α2M‐PL(MA) were not determined.
