Abstract: Synergyistic combinations of β-lactam antimicrobials with aminoglycosides have been uggested as appropriate measure to gain broad antibiotic advantage as well as low toxicity, but the uestion f stability of β-lactam antibiotics under affecting of aminoglycosides indicates the need for clinical trials for ombination therapy.This study measured stability of carbenicillin in different codition with ultroviolet pectrophotometry method, while carbenicillin is combined with gentamycin. Results showed that the stability of carbenicillin is affected hardly by the presence of gentamycin, that carbenicillin is stable both in 0.9% sodium chloride injection and in 5% Dextrose injection. However, when carbenicillin is synergistic with Vitamin C and with gentamycin in two kinds of injections mentioned above, it is more stable in 0.9% sodium chloride than in5% Dextrose injection.