The Bio Basic Carbenicillin Disodium Salt with a composition of 95-100% (2S-(2.alpha., 5.alpha., 6.beta.)) -6-(carboxylatophenylacetami do)-3, 3-dimethyl-7-oxo-4-thia1-azabicyclo(3.2.0)heptane-2- carboxylate, is suitable for use in laboratory use only. It can be stored at a temperature of 2 to 8 degree C.