We have compared the effects of gelatin, low molecular weight hydroxyethyl starch (HES) or albumin on tests of haemostasis and on the thrombelastogram in 42 ASA I patients undergoing total hip or knee replacement. Patients were allocated randomly to receive one of the three blood substitutes to obtain moderate intraoperative haemodilution. Blood loss and packed red cell infusion was the same in each group. A greater amount of gelatin was given (1.5 times the measured blood loss) because of its shorter half-life. There was a statistically significant but clinically negligible decrease in platelets count, prothrombin time and fibrinogen, and an increase in bleeding time in all groups. Platelets were slightly but significantly lower after HES. Haemodilution was comparable between groups. TEG showed a state of hypercoagulability in the gelatin group with a significant decrease in r, r + k and an increase in alpha angle.