One full-term newborn infant and 2 premature ones were treated with cefotaxime for the treatment of suspected sepsis and umbilical suppurative inflammation. Pathogenic organisms could not be identified in all cases. A good result was obtained with the case of suspected sepsis. But the other 2 cases were not evaluable because underlying diseases such as massive pulmonary atelectasis or respiratory distress syndrome masked the effects of this agent. Serum levels of cefotaxime in 3 of the 4 cases were determined with bioassay. Time courses of the serum levels in 2 of them resulted in peculiar biphasic disappearance curves. This fact implies the possibility that desacetylation of cefroxime proceeds also in newborns as in adults and that desacetyl metabolite accumulates in the body owing to the premature function of the neonatal kidney.