The determination of amoxycillin using a flow-injection method with spectrophotometric detection is described. The method is based on the measurement of the intrinsic absorbance of amoxycillin fixed on Sephadex QAE A-25 resin. The determination of amoxycillin, using optimized variables, was carried out for three different sample volumes (300, 600 and 1200 μl), giving linear calibration graphs over the ranges 2-50, 1-25 and 0.5-15 μg ml−1, respectively. The relative standard deviations were ≤ 1%. The proposed method allows a very sensitive, rapid, and simple determination of amoxycillin in the presence of other penicillins, antibiotics, additives and excipients, without performing a prior separation or derivatization. A noticeable increase in sensitivity and lower detection limits can be easily achieved by increasing the sample volume injected. The method was successfully applied for the determination of amoxycillin in pharmaceutical formulations.