1. Negatively charged phospholipids promote initiation of the contact activation system in the blood coagulation. 2. Neutral phospholipids were unable to activate this system. 3. The activation is inhibited by beta 2-glycoprotein I at physiological concentrations. 4. The results raise the question whether people with low concentration of beta 2-glycoprotein I are more easily exposed to blood coagulation defects, such as disseminated intravascular coagulation, than those with normal concentration of beta 2-glycoprotein I.