Procalcitonin (PCT) is a protein consisting of 116 aminoacids with molecular weight 13 kDa. It is encoded by CALC-1 gene. According to the basic and clinical research results PCT appears to be a highly sensitive and specific marker reflecting severity of the systemic inflammatory response to infectious stimuli. Despite the investigation focused on CALC-1 gene, little is known about the biology of PCT and cellular sources of PCT during inflammation. One of the possible sources may be human peripheral blood mononuclear cells. PCT is an indicator of bacterial infections minimally stimulated by viral infections, autoimmune diseases and tumors.