On the occasion of clinically indicated lumbar of cisternal punctures in 19 newborn and premature babies treated with Cefotaxime 33 CSF-levels of Cefotaxime (CTX) and it's metabolite Desacetyl-Cefotaxime (D-CTX) were measured by means of HPLC. 6 of the 19 infants suffered from meningitis. The highest CTX-levels were found 2 to 4 hours after the last infusion of CTX (50 or 100 mg/kg within 20 min, each 12 hours). Patients with meningitis showed CTX-levels between 20 mg/l and less than 0.5 mg/l (limit of detection), those without meningitis between 11 mg/l and less than 0.5 mg/l. Because of the widely scattered CTX-levels any dependence from CTX-dosage or from degree of meningeal inflammation is not to be shown. With two exceptions D-CTX-levels ranged from 2 to 20 mg/l. Up to 9 1/2 hours after the last CTX-dose no clear decrease of the D-CTX-concentration in CSF may be seen. On the other hand, D-CTX-levels are also widely scattered. Nevertheless D-CTX apparently stays significantly longer in the CSF than CTX does. An influence of meningeal inflammation on the D-CTX-levels can not be observed. In E. coli- and in Klebsiella-spp. The geometrical means of MIC are found to range below 0.5 mg/l for CTX and below 2 mg/l for D-CTX. Therefore CTX might be recommended for treatment of meningitis caused by these germs. This recommendation may be supported by some reports about good clinical results, from our unit and from the literature as well.