Pharmacokinetic and bacteriological studies were carried out on sulbactam/ampicillin (SBT/ABPC) in the field of pediatrics. The results obtained are summarized as follows: 1. A total of 248 clinical isolates were employed to determine minimum inhibitory concentrations (MIC) of SBT/ABPC against various bacterial species. SBT/ABPC showed stronger antibacterial activity against Gram-positive organisms than against Gram-negative rods. 2. The peak serum level of ABPC was about twice as high as that of SBT, and serum levels of ABPC and SBT declined with time. Both drugs showed almost the same trend of changes in concentrations. The half-lives of both drugs were about 1 hour. 3. The urinary recovery rates over 6 hours after administration were 52-80% for ABPC and 70-73% for SBT. 4. The effect of SBT/ABPC on coagulation system was examined. No case showed changes in PT, APTT, TT and HPT before and after administration of SBT/ABPC. Platelet aggregation during administration of SBT/ABPC was slightly faster than that before administration, suggesting that SBT/ABPC had no effect on platelet aggregation. Generally speaking, it seemed that SBT/ABPC was a safe antibiotic for bleeding. 5. The effect of SBT/ABPC on the intestinal bacterial flora in experimental animals was examined. Escherichia coli, Enterococcus faecalis, Bacteroides fragilis and Bifidobacterium breve were reduced after administration of SBT/ABPC, suggesting that the intestinal bacterial flora was affected by the administration of SBT/ABPC more greatly than by the administration of ABPC alone. In a similar investigation being made with clinical cases, both aerobes and anaerobes showed great changes. Concentrations of the drugs in feces increased with increasing dosage, resulting in greater changes of the intestinal bacterial flora. Thus, the total number of aerobes and anaerobes was reduced. No diarrhea was observed in any subjects examined. From the above results, it appeared that SBT/ABPC was a safe and useful antibiotic for various bacterial infections in the field of pediatrics.