Cefazolin (7 mg/kg) was administered to 11 children with renal insufficiency and to ten children on hemodialysis. The serum half-life of the drug was progressively prolonged as glomerular filtration rate fell. The serum half-life of cefazolin was variably prolonged in those children on hemodialysis, but their serum levels of cefazolin had dropped by 35 to 65% during dialysis. Most had no measurable level prior to the next dialysis. Dosage recommendations are made for both groups of patients.