The concentrations of five cephalosporins and amoxicillin in breast milk were studied in 42 voluntarily participating lactating mothers using standard assay methods. Each mother received one single dose of 1 g of either an orally or intravenously administered antibiotic. Amoxicillin, cephalexin, and cefadroxil were given orally, and peak milk concentrations averaged 0.81 +/- 0.33 microgram/ml at 5 hours, 0.50 +/- 0.23 microgram/ml at 4 hours, and 1.64 +/- 0.73 microgram/ml at 6 hours, respectively. Cephalothin, cephapirin and cefotaxime were given as an i.v. bolus injection, and peak milk concentrations at 2 hours averaged 0.47 +/- 0.14 microgram/ml, 0.43 +/- 0.16 microgram/ml and 0.32 +/- 0.09 microgram/ml, respectively. The high concentrations of cefadroxil can be explained by its low rate of elimination and higher fat solubility. Milk/serum ratios for all antibiotics were increasing as serum concentrations were diminishing, especially with cephalothin and cephapirin whose serum concentrations are rapidly declining. The significance of bactericidal concentrations in breast milk remains to be evaluated.