Twenty-nine of 54 uremic patients had low levels of protein C measured as anticoagulant activity, contrasting with normal levels measured as amidolytic activity or antigenic concentration. We demonstrate that this discrepancy is due to the presence of a soluble plasma inhibitor that interferes specifically with the anticoagulant activity of activated protein C. The inhibitor does not interfere with other coagulation assays. It is resistant to diisopropylfluorophosphate, high temperatures and repeated freezing and thawing. It can be dissociated from protein C by anti-protein C antibodies or by dialysis in vitro and in vivo. It binds to positively charged resins and can be eluted with high salt concentrations without losing its inhibitory capacity. The inhibitory effect is correlated with plasma creatinine levels and fluctuates with time.