Forty-eight patients scheduled for elective cardiac surgery were randomly assigned to receive aprotinin in the following doses: 0.2 mg kg−1 (group A), 0.7 mg kg−1 (group B), 1 mg kg−1 (group C) and 1.4 mg kg−1 (group D). Clot formation was analysed by thromboelastography immediately after induction of anaesthesia and again 30 min after administration of aprotinin. Rate of clot formation was assessed using R (reaction time = rate of initial fibrin formation), K (clot formation time = rate of fibrin build-up and cross linking) and angle of clot formation (denoting speed at which solid clot forms). Strength of the clot was assessed by maximal amplitude of clot formation (MA)and % lysis after 30 and 60 min. Significant reduction of R and K times and increase in angle of clot formation was observed in groups A and B. This effect was not apparent in the other groups. In group D, an increase in R time was noted. These findings indicate a dose dependent effect of aprotinin on rate of clot formation with an earlier clot formation at low doses.