Alginates are used as haemostats in wound dressings. They act as calcium ion (Ca) donors as they contain mannuronic (M) or guluronic (G) groups with a high Ca content. This study compared the effects of calcium and zinc containing alginates and non-alginate dressings on blood coagulation and platelet activation to determine which was the best haemostat. We showed that alginate materials activated coagulation more than non-alginate materials. The extent of coagulation activation was affected differently by the alginate M or G group composition. It was demonstrated that alginates containing zinc ions had the greatest potentiating effect on prothrombotic coagulation and platelet activation.