Summary. Serum granulocyte-colony stimulating factor (G-CSF) was measured with an ELISA method in patients with acute bacterial and viral infections, or with and atypical pneumonia. Before initiation of antibiotic treatment, G-CSF was found to be significantly increased (799 ± 1501 ng/1) in sera from 34 patients with and acute bacterial infection compared with the 27 patients with the 27 patients with viral infection (58 ± 34 ng/1; P < 0.001) and with the eight patients with an atypical pneumonia (60 ± 33) ng/1; P < 0.001). No significant difference in G-CSF levels was seen between gram-positive and gram-negative bacterial infections. In septic shock, increased G-CSF levels were seen both in patients with leucocytosis and leucopenia. In uncomplicated bacterial infections, both G-CSF and IL-6 were increased on day 0, and decreased rapidly after initiation of antibacterial therapy and before the patients became afebrile. In bacterial infections on day 0, G-CSF levels correlated with mononuclear cells (rs=−0.62, p < 0.001), IL-6 (rs= 0.40, P < 0.05 and S-MPO (rs=−0.5, P < 0.01). In viral infections, G-CSF was correlated with mononuclear cells (rs= 0.041, P < 0.05), White blood cell counts (rs= 0.56, P < 0.01), neutorphils (rs= 0.41, P < 0.05) and CRP (rs= 0.47, P < 0.05). We conclude that G-CSF is rapidly rised in the blood in acute baterica infections but not in acute viral infections or in infections with Mycoplasma pneumonia. Our results also support the theory that G-CSF is involved in the mechanisms of mobilization of neutrophils into the peripheral circulation.