Electron micrographs of isolated human α2M-molecules, obtained by the negative contrast technique, revealed morphologically homogenous structures resembling a graceful monogram of the two letters H and I. The modal values for the length and width of the α2M particles were 170 A and 100 A, respectively. Purified rabbit αmacroglobulins contained about 80% α1M- and 20% α2M-globulins. The isolated rabbit α1M- and α2M-molecules were morphologically indistinguishable from one another and from human α2M-molecules.
Preliminary immunoprecipitation studies demonstrated that the two rabbit αM-globulins were antigenically different. Sedimentation constant determinations gave s20, w values of 18.8 and 18.2 for rabbit α1M and α2M, respectively.