Calcium alginate (Kaltostat, Cair Ltd) is a new absorbable material for topical haemostasis in surgery. The possible mode of action, release of calcium ions in exchange for sodium was investigated in human blood. Calcium release was measured in 15 mg samples of calcium alginate placed in 20 ml of 0.9% saline, for intervals of 1, 3 or 10 minutes. To assess the effect on platelets, 3 mg of calcium alginate or surgical gauze were added to 5 ml of Heparinised (100 units) fresh blood for 2 minutes and platelet counts then made using plain blood as a control. Finally using a thrombelastograph, the activation of whole blood coagulation was assessed after a 2 minute contact with 3 mg of calcium alginate, surgical gauze or no additive as control. When calcium alginate was placed in saline, 26% of calcium ions were released in 1 minute giving a calcium ion concentration of 4.62 t 0.02 mmol/L, with only slight further release after 10 minutes to 4.82 ± 0.004 mmol/L. There was a corresponding decrease in sodium ion concentration. Adding calcium alginate to whole blood reduced the platelet count from a control value of 248 i 16 × 109/L to 222 f 15 × 109/L (p< 0.05) compared to 241 ± 15 × 109/L for surgical gauze. Similarly calcium alginate shortened whole blood coagulation time from 17-7 i 1.0 minutes control, to 12.9 ± 1-32 mins (p< 0.001) compared to 15.0 ± 1.5 mins (p< 0.02) for surgical gauze. Calcium alginate rapidly releases calcium ions in exchange for sodium on contact with blood stimulating both platelet activation and whole blood coagulation, significantly more than simple contact activation by surgical gauze.