To determine the effect of recombinant human granulocyte-colony stimulation factor (rhG-CSF) on the immune system, serum immunoglobulins, lymphocyte subsets, and serum cytokines were analyzed in eight pediatric patients with aplastic anemia (AA) during 8-week rhG-CSF therapy. The rhG-CSF was administered either subcutaneously (200 micrograms/m2 x 4 weeks, followed by 400 micrograms/m2 x 4 weeks) or intravenously (400 micrograms/m2 x 4 weeks, followed by 800 micrograms/m2 x 4 weeks). In response to rhG-CSF therapy, neutrophil counts exceeded the pretreatment counts by twofold during the first week except for one case that did not attain twofold increase until day 41. While serum IgG and IgA were not affected, serum IgM was elevated during treatment in six of the eight cases to more than 1.2-fold basal levels (P < 0.04); however, there was no increase in serum interleukin (IL)-6 and interferon-gamma levels. On the other hand, CD56 positive NK cells significantly dropped from 7.7% to 4.5% (P < 0.02). These results indicate that systemic administration of rhG-CSF affects not only the neutrophil count, but also serum IgM levels and the natural killer cell population in patients with AA.