To further elucidate the dependence of innate defense mechanisms against microorganisms on age, the superoxide anion production (ferricytochrome c reduction) of resting and stimulated (zymosan treated serum) neutrophils obtained from newborns (cord blood) and adults (venous blood) was investigated. Resting newborn neutrophils produced significantly more superoxide than adult neutrophils (p<0.05, t-test) with 8.93 + 4.01 and 5.82 ± 4.34 nmole/5 × 106 cells/30 min, respectively. There was no difference between stimulated noutrophils from newborns and adults and between resting and stimulated neutrophils from the mothers and other adults. The quotient “stimulated/resting” neutrophils was 3.7 in newborns and 6.0 in adults. The meaning of the different quotients with respect to the miorobicidal capacity of newborn neutrophils remains to be elucidated in future.