Neonates are predisposed to serious infections such as meningitis, probably due to their immature host reaction to the pathogens. We have studied the intrathecal immune response in 2 newborns with Flavobacterium meningosepticum meningitis. They showed a significant elevation of immunoglobulin indices ( >1.10), also after the CSF had become sterile with a normalized cell count. In addition, an intrathecal increase and subsequent decrease of both C3dg and TCC (terminal complement complex) were observed in 1 patient. We conclude that immunoglobulin production and complement activation may occur in neonatal CSF.