The aims of this study were, to determine the pharmacokinetic parameters of clarithromycin and to study the passage of the drug into breast milk. Twelve patients (age 24 to 38; weight 44 to 83 kg), suffering from puerperal infections, were treated orally with 250 mg clarithromycin b.i.d. for 6 days. Samples of blood and breast milk were taken at timed intervals. The specimens were assayed for clarithromycin and its active metabolite 14-hydroxy-clarithromycin by HPLC and electrochemical detection. Serum concentrations of clarithromycin in the investigated patients were higher than those reported in healthy volunteers. The mean peak concentrations of clarithromycin and 14-hydroxy-clarithromycin in breast milk were about 25%, and 75% respectively of the corresponding serum concentrations. All patients recovered within 2 to 4 days and no drug-associated side effects (eg. gastrointestinal) were noted. Clarithromycin appears to be an appropriate antibiotic for the treatment of puerperal infections and (because of its considerable concentrations in breast milk) for puerperal mastitis as well.