Interleukin (IL)-12 is a 70-kDa cytokine comprised of two disulfide-linked proteins (p35 and p40) and is essential for the initiation of effective immune response. Granulocyte-colony stimulating factor (G-CSF) affects the balance in the production of anti-inflammatory cytokines. We investigated the serum IL-12 p40 and IL-12 Mix (p40 and p70) production in 28 patients with B-cell lineage non-Hodgkin's lymphoma (NHL) treated with chemotherapy (e.g., CHOP regimen) with or without G-CSF administration and eight healthy volunteers. We found that serum levels of IL-12 p40 (191.2 +/- 150.0 pg/mL) and IL-12 Mix (277.4 +/- 274.5 pg/mL) in the patients before chemotherapy were higher than those in the healthy volunteers (IL-12 p40: 76.4 +/- 25.3 pg/mL, IL-12 Mix: 48.5 +/- 33.4 pg/mL) (P = 0.04 and 0.02, respectively). Next, we examined the serum IL-12 p40 and IL-12 Mix levels in nine patients receiving chemotherapy with administration of G-CSF (CG group, n = 9) and without G-CSF (C group, n = 9). Serum IL-12 p40 and IL-12 Mix levels were decreased on 10 d after chemotherapy in both groups, and those in CG groups were significantly lower than those in C group. These results indicated that administration of G-CSF decreased serum IL-12 p40 and IL-12 Mix levels. Overall survival (OS) at 24 months was not significantly different in the two groups (58.3% in group C vs. 80.0% in group CG, P = 0.67). However, the survival rate of patients at clinical stages III and IV in CG group (n = 6, 66.0%) was significantly better than that of patients in C group (n = 4, 25.0%) (P = 0.02). Long-term administration of G-CSF appears to influence the survival rate by reducing immunosuppressive IL-12 p40 production.