Procalcitonin was recently put forward as a diagnostic tool of systemic bacterial infection and sepsis in children and adults. The value of this marker during the first days of life is controversial. In analysis among 345 neonates procalcitonin serum concentration during the first 12 hours of life is higher than those in adults. None of analyzed perinatal complications affected procalcitonin serum concentration during the first 12 hours of life. Lower calcium levels are correlated with procalcitonin serum concentration elevation in non – infected group. PCT serum concentration during the first 12 hours of life plays an important role in diagnosing early-onset clinical sepsis and sepsis. In congenital pneumonia the PCT serum concentration wasn’t higher than those in non-infected group. Higher, but not significantly, serum procalcitonin concentration was observed in Gram-negative sepsis than in Gram-positive sepsis. There was no correlation between serum procalcitonin concentration and sepsis complications like multiorgan dysfunction, septic shock and death. PCT serum concentration sensitivity is higher compared to WBC, PLT, CRP concentration sensitivity. No significant differences in expression of human leucocyte antigen (HLA-DR), CD14 and CD16 on monocytes between groups with infection and non-infected neonates were observed. There was also no correlation between PCT concentration and expression of human leucocyte antigen (HLA-DR), CD14 and CD16 antigens on monocytes.