Procalcitonin (PCT), a glycoprotein consisting of 116 amino acids, has been proposed as a new marker of severe infection. The site of production under this condition remains unknown. The serum PCT concentration is determined by an immunoluminometric assay of 40 microliters serum or plasma requiring approximately two hours. Elevations of PCT are for instance associated with levels of lipopolysaccharide and the cytokines TNF-alpha and IL-6. Bacterial, parasitic or fungal infections developing septic complications in contrast to local infections, often show values exceeding 2 ng/ml. The specificity of the parameter in this context increases with its concentrations. Therapeutic actions that confine the infection locally are reflected by a decrease of the PCT value. PCT may be elevated within the first days after extended surgery or polytrauma, in some malignancies, heat-stroke and during treatment of some hematologic diseases without an existing sepsis or severe infection. Previous studies indicate certain benefits of PCT compared to traditional markers of inflammation or sepsis, where the ability to indicate a generalized infection is the primary advantage.