Dependence of the citrate human blood plasma clotting kinetics on free calcium concentration under its titration with calcium has been studied in vitro. Activation of factor XI is shown to be independent on calcium, while of thrombin concentration increases non-linearly at calcium concentrations in the range 0.2-0.3 mM. Kinetics of the thrombin generation fits well by the exponential function. Power indexes of the exponents rise steeply as calcium concentration increases from 0.2-0.5 mM and reach plateau at higher concentrations. At free calcium concentrations under 0.2 mM the thrombin level does not increase and remains lower than 10 pmol/ml as seen by our measuring system whose sensitivity threshold is surely less than 10 pmol/ml. Thus, the blood coagulation system behaves in a threshold manner under changes of calcium concentration. The threshold concentration of free calcium is equal to 0.2 mM.