Bactericidal activities of neutrophils occur by two distinctive mechanisms that are oxygen-dependent and -independent. Human cathelicidin antimicrobial peptide 18 (hCAP18), also known as LL-37/FALL-39, is a neutrophil-specific granule protein. We compared the content of hCAP18 and neutrophil gelatinase-associated lipocalin (NGAL), another neutrophil-specific granule protein, in neutrophils of both neonates and adults by flow cytometry. The percentage as well as fluorescence intensity ratio of hCAP18 and NGAL expression in neonate neutrophils were significantly lower than in adults. Expression of hCAP18 in monocytes, however, was not significantly different between neonates and adults. Both hCAP18 and NGAL expression increased in an age-dependent fashion. Plasma concentration of these peptides measured by enzyme-linked immunosorbent assay was not significantly different between neonates and adults. Oral intake of 1alpha hydroxy vitamin D(3) (1alpha(OH)D(3)) in rickets patients for 4 weeks significantly increased hCAP18 expression in neutrophils compared to age-matched healthy controls without 1alpha(OH)D(3), indicating the potential of vitamin D(3) as a regulator of the innate immune response of neonates.