The objective of the present study is to investigate the effect of higher-molecular polyanionic compounds on the blood coagulation in vitro and on the vitality of U937 cells. The test substances are sulfated hyaluronic acid (S-Hya-15), carboxymethylated sulfated hyaluronic acids (CMS-1, CMS-2) and peat humic acids (THS-A1, THS-A2). As starting substances for the preparation of the sulfated polyanionic compounds, hyaluronic acid and its carboxymethylated derivative were used, respectively. The sulfated compounds were obtained by sulfation with a sulfur trioxide/dimethyl-formamide complex. Humic acids were prepared from the peat of the Altteich peatland (Saxony) using conventional alkali extraction and acid precipitation methods. The anticoagulant effect of the test compounds was proven in human citrated plasma using the group assays thrombin time (thrombin-fibrinogen reaction), activated partial thrombo-plastin time (intrinsic system) and prothrombin time (extrinsic system). The thrombin time in fibrinogen solution was also determined. S-Hya-15 and the higher-sulfated CMS-1 exerted the strongest inhibitory effect on coagulation, followed by the low sulfated CMS-2. Compared with the sulfated compounds, humic acids showed a significantly weaker effect in the group assays. The anticoagulant effect of all test compounds was antagonizable with protamine sulfate. Investigations on the cytocompatibility of the test compounds revealed generally low cytotoxicity. After 24-hour exposure of U937 cells to the sulfated test compounds, the half maximum cytotoxic concentration was found to be within the milligram range (>10 mg/ml). After 96-hour exposure, S-Hya-15 and CMS-1 (12.8 mg/ml) revealed a significant antiproliferative effect. The membrane toxicity of the compounds proved to be so low that the half maximum [3H]arachidonic acid release could not be calculated. Therefore, the RC5, i.e. the substance concentration causing 5 % of the maximally possible arachidonic acid release, was used as the criterion for membrane injury. By this means, after 24-hour exposure of the test compounds at 37°C the following ranking for membrane toxicity was found: THS-A1 (RC5 = 1.5 mg/ml) > THS-A2 > S-Hya-15 > CMS-1 = CMS-2 (RC5 > 30.0 mg/ml). Taken together, the carboxymethylated sulfated hyaluronic acids CMS-1 and CMS-2 proved to be highly effective and cytocompatible coagulation inhibitors. Their anticoagulant activity clearly depends on the sulfatation degree (CMS-1 > CMS-2), but does not exceed the effectiveness of sulfated hyaluronic acid.