Recombinant activated factor VII (NovoSeven; Novo Nordisk A/S, Måløv, Denmark) is an effective drug for treatment of bleeding in patients with haemophilia A or B and inhibitors. Little is known about physiological conditions influencing the efficacy of recombinant activated factor VII. We investigated the in-vitro effects of pH, temperature, and haemodilution on the activity of recombinant activated factor VII. Samples from eight healthy volunteers were spiked with recombinant activated factor VII (final concentration 1.7 microg/ml) and adjusted to pH 6.0, 6.5, 7.0, and 7.4 or analysed at 30, 33, 37, and 40 degrees C, or diluted 0, 10, 20, 40, and 60% with dextran before analysis. Samples were analysed as rotational thromboelastometry in whole blood (clotting time, clot formation time, and maximum clot firmness) with and without Innovin (tissue factor), and as factor VII coagulant activity in plasma. Significant effects of pH were observed for clotting time, clot formation time, maximum clot firmness, and factor VII coagulant activity in the direction of longer clot formation times and less firm clots with decreasing pH. Temperature had significant effects on clotting time, clot formation time, and factor VII coagulant activity, but no effects on maximum clot firmness indicating that lower temperatures increase clot formation times without affecting clot firmness. Haemodilution had significant effects on clot formation time, maximum clot firmness, and factor VII coagulant activity, but no effects on clotting time indicating that haemodilution does not affect clot formation, but the clot formed at high haemodilution may not be so firm. In conclusion, the activity of recombinant activated factor VII was affected in vitro by pH, temperature, and haemodilution. Additional studies are necessary to demonstrate that these conditions also affect the efficacy of recombinant activated factor VII therapy in vivo.