Twelve normal volunteers in the fasting state were given 1000 mg cefaclor, and the serum and urine concentrations over 8 h and 24 h respectively were measured. The average peak serum concentration was 34.6 +/- 7.8 mg/l, this value being reached after 65.2 +/- 11.1 min; the half-life was 42.5 +/- 8.3 min. In another six volunteers the absorption of 500 mg of 'cefaclor following administration in the fasting state and after a test breakfast was studied. The peak serum concentrations after administration in the fasting state were 16.1 +/- 3.2 mg/l, and after a meal 12.5 +/- 1.9 mg/l; the areas under the curve did not differ. The low recovery rate of cefaclor in urine observed in this series of investigations could be partly explained by the inactivation of the substance in urine. Cefaclor was administered therapeutically to 23 patients, most of whom were suffering from bronchopulmonary infections and chronic pyelonephritis. The results of therapy were good in four patients, satisfactory in 13 patients and unsatisfactory in three patients. Intolerance was rare.