The effect on hemostatic parameters of two non-ionic contrast media, iopentol and iohexol in concentrations of 350 mg I/ml, were tested in vitro. The new medium iopentol had similar effects to those of iohexol, both with respect to the intrinsic coagulation system and platelet aggregation. As part of a human pharmacologic study the effects of iopentol on hemostatic parameters were tested in 8 volunteers in vivo. Iopentol induced a slightly decreased coagulability and aggregability in venous blood. It was concluded that the effects were of very little consequence for hemostasis as a whole.