Fundamental and clinical studies were made on piperacillin (PIPC) and the results were obtained as follows. Serum and uterine tissue concentrations of PIPC were obtained from 36 to 215 minutes after intravenous single shot of 2 g of PIPC. The cervix uteri, endometrium and corpus uteri showed the highest antibiotic levels of 38.0, 43.0 and 33.0 mcg/g, respectively, at 65 minutes after injection, and oviduct and ovary showed the highest level of 31.5 and 28.5 mcg/g at 36 minutes. Its concentrations were sufficiently effective against the major pathogens (Gram-negative bacilli and anaerobes) demonstrated in the field of obstetrics and gynecology. PIPC was administered 6 patients, including 3 of pelvic peritonitis (isolated organism was E. coli 1), 2 of acute endometritis (Klebsiella sp. 1, Peptococcus sp. + Bacteroides sp. 1) and 1 of acute adnexitis, in a dosage of 1 or 2 g twice or 3 times a day for a period of 5 to 8 days by intravenous administration or intravenous drip infusion. Clinical response was obtained excellent in 1 and good in 5. No adverse reaction as observed in any of the cases treated with PIPC, nor was there any marked changes in the laboratory findings.