Ceftazidime in the breast milk of 11 puerperal women were determined by bioassay. Each patient received 2 g of ceftazidime intravenously every 8 h for 5 days. The mean (+/- standard deviation) ceftazidime concentrations in the milk were: 3.8 +/- 2.0 (before the next dose), 5.2 +/- 3.0 (1 h after), and 4.5 +/- 1.7 micrograms/ml (3 h after). Ceftazidime was excreted in breast milk at relatively constant levels between days 2 and 4 of therapy.