The stability-time profiles of the active ingredient of five generically equivalent brands of penicillin V potassium for oral solution, and of five generically equivalent brands of ampicillin for oral suspension, were studied. Three controlled conditions were employed-refrigerated, room and elevated temperature-and all the samples were assayed chemically for drug remaining at specific time intervals after reconstitution. The results showed that considerable variations in the initial concentrations of active component existed among the various ampicillin and penicillin products. In one penicillin product the official content requirement was not met. The data also showed that although the labels on each of the commerical penicillin products tested indicate that the reconstituted products may be stored in a refrigerator for 14 days without significant loss of potency, only one penicillin product still met 90% of label claim (minimal potency requirement of the United States Pharmacopeia for dry powder). All ampicillin products tested were stable when stored at the conditions recommended by the manufacturers, but the trihydrate forms exhibited greater stability than the anhydrous forms, probably because of the more rapid dissolution rate of anhydrous ampicillin.