Investigation of the effects of carrageenans and alginic acid on the coagulation and platelet aggregation systems was the aim of this study. The highest anticoagulant and fibrinolytic activities were found in lambda-carrageenan and Grateloupia dichotoma carrageenan. Antiplatelet activity of carrageenans showed that ADP-induced aggregation was inhibited slightly by tau-carrageenan whereas lambda- and kappa-carrageenans were not effective. Ristocetin induced aggregation was inhibited by all the carrageenans tested. Alginic acid did not show any activity against coagulation, fibrinolytic and platelet aggregation systems.