We reported that recombinant human soluble thrombomodulin (rhs-TM) is effective for disseminated intravascular coagulation (DIC) in vivo, in mice and rats. In the present work, we investigated the effects of decreased plasma antithrombin III (ATIII) levels on anticoagulant effects of rhs-TM, as compared to findings with heparin, of which effect is lowered by the decreased plasma ATIII levels in patients with DIC. Rat plasma ATIII levels decreased when we mixed plasma with anti-rat ATIII antibody and the potential of heparin to prolong APTT or PT was markedly diminished. The potential of rhs-TM to prolong APTT and PT was not affected. In rats injected with anti-rat ATIII antibody, plasma ATIII levels decreased immediately. When the rats were infused with tissue factor (TF), DIC was induced. At doses of rhs-TM and heparin which were equally effective at inhibiting the decrease in platelet count and fibrinogen level in control rats treated with TF, only rhs-TM remained effective in preventing DIC in rats with reduced ATIII levels. Heparin was not effective when administered to these rats with reduced ATIII levels. Therefore, rhs-TM effectively inhibits coagulation independent of ATIII levels, in contrast to heparin, which depends on the ATIII level.