Abstract Crystals of the plasmin Fc fragment of human immunoglobulin G have been examined in the electron microscope at various stages during dissociation and recrystallization. Two characteristic shapes are recognized in the micrographs. One is a double, concentric annulus in which the outer annulus is seen to consist of 12 masses disposed radially and attached to the inner annulus by short stalks. The centres of maximum density of the concentric annuli lie at radii of 40 A and 75 A respectively. The other shape seen frequently in micrographs is a rod-shaped object appximately 150 A in diameter and varying in length from 200 to 1000 A. The rods are composed of segments 70 A, each divided longitudinally into 35 A sections. It is believed that the segments represent side views of the annuli. A model of Fc consistent with these findings and with the present state of knowledge is presented.