In vitro clotting kinetics of citrate human blood plasma under its titration with calcium ions are studied. Contact activation (Factor XIa + plasma kallikrein) is shown to be independent of calcium while non-linear growth of thrombin concentration is demonstrated at calcium concentrations higher than 0.25 mM under conditions of contact activation caused by quartz walls of the measuring cell. Thrombin generation kinetics are well fitted with an exponential function. Power index of the exponential function steeply rises as calcium concentration increases from 0.25 to 0.5 mM and reaches plateau at higher concentrations. At free calcium concentrations under 0.25 mM thrombin level does not grow remaining lower than 30 pM. So, blood coagulation system behaves in a threshold manner under calcium concentration changes. The threshold concentration of free calcium is equal to 0.25 +/- 0.05 mM.