The intrinsic bactericidal capacity of MP and PMN of 16 normal newborn infants and 10 healthy adults against GBS-Ia and III prototype strains was measured in vitro using optimal opsonic (single batch fresh pooled human serum) and leukocyte:bacterial (1:10) ratios. Interstrain variability of intrinsic bactericidal capacity was further evaluated exposing MP and PMN of three normal newborns simultaneously to five different GBS-III strains. Of all cell:bacterial combinations, only newborn PMN were found to have a significantly (p < 0.05) diminished bactericidal index (B.I.) (64.6% ± 8.1%) (mean ± s.e.) vs GBS-III when compared to adult PMN (87.2% ± 4.3%). However, 25% of all newborn PMN were significantly less bactericidal (B.I. ≤ 40%) towards both GBS-Ia and GBS-III by the Moses test for extreme reactions (p < 0.05). Of the three sets of newborn leukocytes confronted with five strains of GBS-III, PMN of one revealed diminished B.I. against two strains, while the other two sets were bactericidally normal against all five strains. In addition to critical deficiencies in opsonic factors, newborns may also have intrinsic cellular bactericidal deficiencies against some GBS strains and this may constitute another risk factor for contracting GBS disease.