CD14, a high-affinity receptor for lipopolysaccharide (LPS), is a glycoprotein expressed on the surface membranes of monocytes/macrophages. We have identified a previously unknown form of soluble CD14, named soluble CD14 subtype (sCD14-ST), that is increased in patients with sepsis. To measure sCD14-ST concentrations in plasma, we prepared anti-sCD14-ST antibodies and developed an enzyme immunoassay (EIA) for this soluble form of CD14. With this assay, quantitative measurements are available within 4 h, and we compared the levels of sCD14-ST in plasma from normal subjects (healthy controls), patients with systemic inflammatory response syndrome (SIRS), and sepsis patients. The level of sCD14-ST in subjects with sepsis was much higher than the levels in subjects with SIRS and the healthy controls. Additionally, when a subject's sCD14-ST level was used as a diagnostic marker for sepsis, the area under the receiver operating characteristic (ROC) curve was 0.817, thereby demonstrating that elevated sCD14-ST levels were a better marker for sepsis than the other molecular markers we tested. sCD14-ST levels also correlated with procalcitonin (PCT) levels and with sequential organ failure assessment (SOFA) scores. Finally, changes in sCD14-ST concentration correlated with the severity of sepsis. Taken together, these results indicate that sCD14-ST is a useful marker for the rapid diagnosis of sepsis and for monitoring the severity of the disease.