Clinical study of sustained release cephalexin (S-6435 and S-6436) was conducted with the following results: 1) Pediatric patients with 1.5 approximately 12 years of age having acute tonsillitis and pharyngitis were orally administered 500 approximately 1,000 mg/day of S-6435 or S-6436 divided into 2 doses after breakfast and dinner. Out of 21 patients receiving the preparations, 19 patients satisfactorily responded to them and 2 fairly did. 2) In only one patient of the 21, rash was observed during the treatment. The side effect, however, disappeared 2 days after the initiation of the treatment, and was considered to be due to underlying disease of the patient. No other side effects were observed. Since it has become our concern to conduct injectable therapy in children, oral therapy should first be recommended to them. However, oral drugs, like a regular cephalexin, requiring q.i.d. regimen are inconvenient in giving the drugs to patients because they have to take the drugs once at midnight. S-6435 and S-6436, however, have no such disadvantages and prove to have as much effectiveness and safety as regular cephalexin has.