The superoxide anion production and the myeloperoxidase activity between polymorphonuclear cells (PMNs) from conventional and germfree rats were compared. Furthermore, the response to recombinant human granulocyte colony stimulating factor (G-CSF) between both groups of rats was also examined. Our present investigation showed that: (1) the basal superoxide anion production by PMNs in germfree rats (2.1 +/- 0.5 nmol/min/1 x 10(6) PMNs) is less than one fourth of that of conventional rats (9.5 +/- 2.9 nmol/min/1 x 10(6) PMNs). (2) the myeloperoxidase activity of PMNs from germfree rats (38.1 +/- 5.2 nmol/min/1 x 10(6) PMNs) is 1.7 times that of conventional rats (22.4 +/- 4.9 nmol/min/1 x 10(6) PMNs). (3) G-CSF administration increased the number of PMN, 12.4 times in the germfree rats compared with 3.9 times in the conventional rats. (4) the PMNs induced by G-CSF do not differ in the superoxide anion producing activity. However the myeloperoxidase activity decreased in the G-CSF induced PMNs of both groups of rats. From our data, the conventional rats could have a more effective killing activity than the germfree rats. Furthermore, germfree rats may be used as a powerful tool for assay of other biological substances relating to immune system and its regulation.