The stability of ampicillin trihydrate oral suspension stored in amber plastic oral syringes was studied. Commercially available ampicillin trihydrate powder for oral suspension was reconstituted according to manufacturer's instructions and drawn into 5-mL amber polypropylene plastic oral syringes. The syringes were divided into groups and stored at -20, 4, 25, 60, or 80 degrees C. Powder from two additional lots was similarly reconstituted and packaged and stored at 80 degrees C only to assess interlot variability. Immediately after reconstitution and at specified times during storage, three syringes at each storage temperature were removed and their contents analyzed for ampicillin trihydrate concentration by a spectrophotometric assay. Samples stored at frozen (-20 degrees C) or refrigerated (4 degrees C) temperature retained at least 90% of the initial ampicillin concentration throughout the 47-day study period. Samples stored at room temperature retained at least 90% of the initial ampicillin concentration for 30 days and exhibited an apparent zero-order degradation rate. Samples stored at heated temperatures (60 and 80 degrees C) exhibited an apparent first-order degradation process, with the concentration of ampicillin decreasing to less than 90% of initial concentration within two hours. Reconstituted ampicillin trihydrate powder for oral suspension is stable for at least 30 days when stored at room, refrigerated, or frozen temperature in the amber plastic oral syringes studied. The expiration dates recommended by the manufacturer for ampicillin trihydrate suspension stored in its original container can also be used for reconstituted suspension stored in these amber plastic syringes.