Our objective was to investigate the in vitro effects of a totally balanced fluid concept on whole blood coagulation. Venous blood from 12 healthy volunteers was diluted by 20% and 40% with a combination of an equal amount of colloid (balanced or unbalanced 6% HES 130/0.4, or 4% gelatin) and crystalloid (balanced or unbalanced Ringer's acetate). Blood samples were analyzed with rotational thromboelastometry (ROTEM®). The initiation of coagulation was delayed in all dilutions except for the 20 vol% gelatin-dilution. In the extrinsic activation test, maximum clot firmness was decreased and clot formation time prolonged after 40 vol% hemodilution with a balanced Ringer's/unbalanced HES combination, more than in the corresponding gelatin hemodilution. In the fibrin-based test, after both 20- and 40 vol% hemodilution with unbalanced Ringer's/gelatin solution, maximum clot firmness was significantly stronger than in the Ringer's/HES-combinations. The combination of balanced colloid and crystalloid has similar coagulation effects in vitro as their respective combination of unbalanced solutions.