Previous studies of human in vivo complement protein levels have only compared data for neonates with that from adult sera. Here, we establish the normal concentration ranges of the following complement regulatory proteins in healthy Brazilian children of different age groups (neonates: 1 month-1 year, 1-6 years and 6-13 years) and in adults: factor H (fH), factor I (fI), C4b-binding protein (C4 BP), properdin and vitronectin. We found that the concentrations of fH, fI, properdin and vitronectin in neonates are significantly lower than in adults. Remarkably, the concentration of C4 BP is below the method resolution (<50 micro g/ml) in 76% of the sera from neonates, while adults presented 199-532 microg/ml of C4 BP in their sera. The concentration of properdin in the sera from neonates and up to 1-year-old children was less than that observed in older children. Adults presented vitronectin levels significantly higher than all the other age groups in the study. No significant sex differences in the concentrations of all the studied regulatory proteins were detected. This study reveals the ontogeny of complement system in greater detail than previously available and may point to the reasons why neonates have higher susceptibility to develop life-threatening pyogenic infections. These reference values will be of use in clinical and laboratory investigations of disorders associated with low levels of these regulatory proteins.