Surface plasmon resonance technology was used to study the binding of activated factor VII to tissue factor, in the presence of divalent metal ions. The binding was calcium-dependent, with the optimum calcium concentration at 5 mM. Only very minimal binding was observed in the absence of added calcium ions and presence of 10 mM EGTA. The effect of the calcium concentration on the apparent dissociation constants for the TF/VIIa complex formation was also investigated. In the absence of calcium ions but in the presence of either magnesium or zinc ions, no significant binding of activated factor VII to tissue factor was detected. However some binding was observed with manganese ions, indicating that manganese ions could partially replace calcium ions to support the formation of the TF/VIIa complex. These results are consistent with studies of the effect of divalent metal ions on the amidolytic activity of the TF/VIIa complex.