Aspoxicillin (ASPC), a new injectable semisynthetic penicillin, was administered to healthy volunteers to elucidate its safety and pharmacokinetics. No abnormalities obviously attributable to ASPC were observed in the examinations covering subjective and objective symptoms, blood pressure, heart rate, respiratory rate, electrocardiogram and body temperature, as well as in hematology, blood chemistry and urinalysis. There were slight increase in LDH (1 case), Al-P (1 case) and blood-glucose levels (2 cases) at 2 or 4 weeks after the administration with intravenous bolus consecutive injection (1 g X 1 or 2/day, 8 to 10 times), but these increases were not considered to be attributable to ASPC. When ASPC was administered by consecutive intravenous bolus injection, there observed no tendency of accumulation of the drug in serum or urine. When 1 g of ASPC was given intramuscular (i.m.), intravenous injection (i.v.) and intravenous drip infusion (d.i.) route, the maximum antibiotic levels in serum reached 25.2 micrograms/ml, 118.2 micrograms/ml and 70.3 micrograms/ml at 0.75 hour, 0.08 hour and 1 hour after the administration, respectively. The biological half-lives of ASPC attained by these 3 different routes were as follows: 1.73 hours (i.m.), 1.65 hours (i.v.) and 1.44 hours (d.i.). In a cross-over test with piperacillin (PIPC), serum levels of ASPC were higher than those of PIPC and half-lives of ASPC were longer. The 8 hours urinary recovery of ASPC was 75.9% after intravenous injection, 74.6% after intramuscular injection and 88.0% after intravenous drip infusion. Most of urinary recovery was excreted within 4 hours after administration regardless of dose levels, frequency of dosing or administration route. Thin layer chromatographic bioautography was conducted with the samples of serum and urine collected from subjects received ASPC. No antibacterial metabolite was observed in the serum obtained 2 hours after administration but a metabolite identified as amoxicillin was detected in a part of urine samples collected after 8 to 10 hours after the injection.