Cefotaxime was given to neonates as treatment of infection in a dose of 25 mg/kg 12 hourly by intravenous injection. Blood samples taken by heel-prick were specially treated to minimize any effect of haemolysis on the hydrolysis of cefotaxime. The mean peak plasma concentrations of cefotaxime on the first, second and third days of therapy were 43, 40 and 52 mg/l, respectively, with mean plasma half lives of 4.0, 2.7 and 3.2 h. The mean peak concentrations of desacetyl-cefotaxime were about a quarter of those of cefotaxime, which is in good agreement with adult studies, but much lower than those previously published in studies with neonates. These results emphasize the care that is necessary in the assay of body fluids and tissues for cefotaxime and desacetyl-cefotaxime if accurate results are to be obtained.