The in vitro effect of recombinant human granulocyte-macrophage colony stimulating factor (rh-GMCSF) and recombinant human granulocyte colony stimulating factor (rh-GCSF) on oxygen free radical (OFR) generation by human neutrophils and blood monocytes derived human macrophages stimulated with phorbol myristate acetate was investigated and compared. The production of OFR by neutrophils and macrophages was time dependent, and the maximum release of OFR by neutrophils and macrophages was measured 90 and 180 min after stimulation with phorbol myristate acetate, respectively. The priming effects or rh-GMCSF and rh-GCSF on OFR production by human neutrophils and macrophages was dose dependent. The maximum generation of OFR by neutrophils occurred when primed with 1,000 U/ml of rh-GMCSF and reached 2.383+0.191 nmol/l05 neutrophils/90 min as compared with 1.072 ± 0.113 nmol/l05 neutrophils/90 min in the unprimed controls. This represents a 122.20% increase in OFR generation (p < 0.001). However, the percentage of maximum increase in OFR production was 57.84 when neutrophils were primed with a concentration of 5,000 U of rh-GCSF/ml. In 72-hour-old human macrophages, much higher levels of OFR production as compared with neutrophils were measured following stimulation with phorbol myristate acetate. The maximum generation of OFR was measured in macrophages primed for 45 min with 500 U/ ml of rh-GMCSF. These cells produced 8.960 ± 2.075 nmol/5×104 macrophage/180 min as compared with 4.563+1.773 nmol/5×104 unprimed macrophages/180 min (p < 0.001). In macrophages primed with rh-GCSF, however, the maximum OFR production was induced by a dose of 5,000 U/ml and reached 6.902+1.463 nmol/5× 104 macrophages/180 min as compared with 4.563 ± 1.773 nmol/5×104 macrophages/180 min in the unprimed controls (p < 0.05). In conclusion, the priming effect of rh-GMCSF on OFR generation by human macrophages and neutrophils was more potent than that of rh-GCSF, both in the extent of augmentation and in the dose required to produce maximum OFR generation.