AbstractWe investigated the capacity of recombinant human granulocyte‐macrophage colony‐stimulating factor (rhGM‐CSF) to enhance the function of neutrophils. Neutrophil function was measured in terms of N‐formyl‐methionyl‐leucyl‐phenylalanine (fMLP)‐induced luminol‐dependent chemiluminescence (LDCL). LDCL of fMLP‐stimulated neutrophils was enhanced up to 4.5 fold following preincubation with rhGM‐CSF. This enhancement depended on the length of preincubation, reaching an optimal level at 120 min. The dose‐response relationship for fMLP‐induced LDCL of neutrophils preincubated with rhGM‐CSF revealed that half‐maximum enhancement was achieved at an approximately 20‐fold higher concentration than that of colony‐forming units in culture‐derived colony formation. These results suggest that differences in dose dependency may be explained by differences in the distribution of receptor(s) for GM‐CSF. This may also enable GM‐CSF to affect the hematopoietic system, which contains cells at various levels of differentiation, th