The dependency of concentrations of Zn2+ and the negatively charged surfaces, phosphatidylinositol phosphate (PtdInsP), sulfatide and dextran sulfate, on the autoactivation of human factor XII, has been studied. While the autoactivation induced by sulfatide, and low concentrations of dextran sulfate, was unaffected by the presence of Zn2+, that induced by PtdInsP and higher concentrations of dextran sulfate was completely dependent on Zn2+: the excess of Zn2+ needed to induce maximal activity with PtdInsP was 12-fold the concentration of factor XII, while with dextran sulfate it was 40-fold. Determination of the Zn2+-binding properties of factor XII revealed that a total of four zinc ions could bind to each factor XII molecule. The first bound zinc ions (Kd 0.1 μM) induced an increase in the intrinsic tryptophan fluorescence of factor XII, while further titration up to a 40-fold surplus resulted in a quenching of the fluorescence. Binding of the zinc ions that caused the quenching had an average Kd of approximately 1 μM, independent of whether it was determined from the fluorescence changes or by equilibrium filtration. Low concentrations of both sulfatide and PtdInsP induced a fluorescence increase similar to that at low concentrations of Zn2+ but, in contrast to sulfatide, higher concentrations of PtdInsP did not induce a quenching in fluorescence. As the Zn2+-independent activating surface (sulfatide) induced quenching in the fluorescence intensity, while the Zn2+-dependent activating surface (PtdInsP) did not, the quenching, whether it was caused by sulfatide or zinc ions, was assigned to a change in the conformation which resulted in a molecular structure of factor XII that could be autoactivated. Association of factor XII in this conformation on the activating surface was suggested to be responsible for the autoactivation.