Asimple and rapidHPLCassay for ampicillinmeasurement in human plasma was developed and validated. 0.25 ml plasma sample was precipitated with 50 µl of perchloric acid, and 100 µl of the supernatant was directly injected into 4.6ï´150mm, SymmetryShield,RP8, 4-µm steel column at room temperature (RT). The mobile phase, 0.05Mdibasic sodiumphosphate buffer (pH = 7.3) and acetonitrile (87:13, v:v), was delivered at 1.0ml/minwith a run time of 9min. Ciprofloxacin (internal standard, IS) and ampicillinwere detected at 3.9 and 8.1 min, respectively, usingWaters 2998 photodiode array detector set at 210 nm. The response was linear over the range of 0.3Â15 µg/ml, and the intra-and inter-run coefficient of variations were ï£ 10.7% and 10.4%, respectively. Extraction recovery and intra- and inter-run bias were ï³ 86% (mean 91%), ï£ 11%, and ï£ 7%, respectively.Ampicillinwas stable in plasma for 24 hours at RT (ï³ 87%), 5 weeks at Â20 0C(ï³ 93%), and after 3 cycles of freeze at Â20C and thaw at RT (ï³ 89%). In precipitated plasma samples, ampicillin was stable for 24 hours at RT (ï³ 93%) and 48 hours at Â20ï°C (ï³ 85%). Stock solution of ampicillin (1mg/ml inwater) was stable for 48 hours at RT (93%) and 5 weeks at -20ï°C(97%).