Transmission electron microscopy of purified αIIbβ3 in the presence of 1 mM Ca²⁺ or 1 mM Mn²⁺. Individual αIIbβ3 molecules were visualized using transmission electron microscopy after rotary shadowing with tungsten. The images could by classified into 3 groups, of which 2 are shown. (A) Closed structures with the tips of the αIIb and β3 stalks touching that were obtained in the presence of 1 mM Ca²⁺. (B) Open structures with the αIIb and β3 stalks separated that were obtained in the presence of 1 mM Mn²⁺. A minority of structures consisting of globular headpieces with no visible stalks were also observed in the presences of both Ca²⁺ and Mn²⁺. Magnification, × 170 000.