Neonatal polymorphonuclear leukocytes (PMNs) are deficient in the expression of the adherence protein C3bi (Mol), and are associated with reduced physiological inflammatory responses. We evaluated the priming and direct stimulating effect of recombinant human granulocyte-macrophage colony stimulating factor (rhGM-CSF) on newborn PMN expression of C3bi (Mol), PMN adherence and PMN aggregation. Cord PMNs were incubated with rhGM-CSF (Amgen 4 × 107U/mg) for 0–15 min, and C3bi surface receptor expression measured by immunofluorescence with CD11b, adherence with nylon wool, and aggregation using a Payton aggregometer. RhGM-CSF (15 min) significantly induced Mol expression: 250 pM/L 129.4 ± 5.3% of C (p < 0.001) 500 pM/L 141.5 ± 4. 1% (p < 0.001), 1,000 pM/L 150.2 ± 1.3% (p < 0.0001). RhGM-CSF (1,000 pM/L × 5 min) followed by A23187 also primed newborn PMNs for increased Mol expression 122 ± 4.5% of C (p < 0.001). Additionally, rhGM-CSF (10 min) induced significant PMN adherence 50 pM/L 117.9 ± 8.3% and 100 pM/L 131.5 ± 5.7% of C (p < 0.04). RhGM-CSF additionally primed newborn PMNs (100 pM/L) for increased adherence following A23187 (107.9 ± 0.6% of C), p < 0.02. Lastly, rhGM-CSF primed newborn PMNs for increased aggregation following FMLP: 100 pM/L, 15 min, 138.1 ± 14.1%, p < 0.0001. Co-incubating murine-antihuman GM-CSF AB 100 μg/ml neutralized 86.8 ± 7.0% of newborn PMN Mol up-regulation. These studies demonstrate that rhGM-CSF primes and directly stimulates newborn PMNs for increased in vitro expression of Mol, adherence, and aggregation. This modulation in PMN mobility may prove to have either a beneficial or negative effect in future in vivo studies.