Long acting amoxicillin granules (L-AMPC), a newly developed presentation of amoxicillin (AMPC), maintains an effective blood level for an extended period. The following results were obtained from fundamental and clinical studies of L-AMPC. Serum and gingiva levels of AMPC were measured in 5 patients at 2 (Patient 1), 3 (Patient 2), 4 (Patient 3), 5 (Patient 4) and 6 (Patient 5) hours. Serum levels were 4.1, 5.2, 3.0, 1.7 micrograms/ml and 1.2 micrograms/ml respectively, and gingiva levels were 1.3, 2.6, 2.1, 1.1 micrograms/g and 1.1 micrograms/g respectively. Twenty-five patients with oral and maxillofacial infections containing maxillary or mandibular osteitis were treated with L-AMPC. Clinical efficacy was assessed according to the criteria of numerical judgement established by the Japanese Society of Oral Surgery (1973) and also by the global evaluation of the clinician. In numerical judgement there were 1 excellent case, 19 good cases and 5 poor cases (efficacy ratio; 80%). The clinician's assessment was 4 excellent cases, 17 good cases, 3 fair cases and 1 poor case (84%). Although 2 slight side effects, diarrhea and black hairy tongue were observed, both treatments were completed without problem. L-AMPC therapy (b.i.d.) showed good prolonged tissue levels and satisfactory clinical results.