The concentrations of both polyP and Ca2+ influence fibrin clot turbidity. The final turbidities of clots formed from fibrinogen/thrombin mixtures were quantified as described in “Measurements of clot turbidity.” Reactions contained 2.6 mg/mL citrate-free fibrinogen, which was preincubated for 15 minutes with CaCl2 and the indicated concentrations of polyP (x-axis), after which clotting was initiated with 1 nM thrombin. Ca2+ concentrations were 0 (●), 2 mM (▼), 2.5 mM (□), 3 mM (▼), and 5 mM (◇). Data are mean plus or minus SE (n = 3).