We evaluated the effect of recombinant human granulocyte colony-stimulating factor (rhG-CSF) given after myelosuppressive chemotherapy in 15 cancer patients. No severe neutropenia (absolute neutrophil count, ANC < 0.5 x 10(3)/microL) was noticed in 10 rhG-CSF primary prophylactic patients, but was noticed in two of five rhG-CSF secondary prophylactic patients. Neutrophilia characterized by shift to the left occurred within 24 hours after starting rhG-CSF prophylaxis. Thereafter, conversion to normal level occurred within 24 hours. The peak of neutrophilia occurred earlier in the primary group than in the secondary prophylactic group. The detection of myeloperoxidase (MPO) using flow cytochemistry blood autoanalyzer (TechniconR H * 1) was evaluated as mean peroxidase index (MPXI). Leukocyte alkaline phosphatase (LAP) using the method of Kaplow (Am J Clin Pathol 39:439-449, 1963) was recorded as LAP score. There was a statistically significant elevation of MPXI in the primary group over the secondary prophylactic patients. The LAP activity was in normal range. There was a slightly decreased red blood cell (RBC) count, hemoglobin (Hb), and platelet count. In conclusion, rhG-CSF induced neutrophilia with efficient enzymatic activity. These findings demonstrate the value of rhG-CSF in patients receiving chemotherapy. MPXI and early neutrophilia may serve as a potential biomarker of therapeutic efficacy of rhG-CSF.