Crystalline sulfuric, di-nitric, mono-hydrochloric, di-hydrochloric, and di- and sesqui-orthosphosphoric acid addition salts of 7-[ alpha -(2-aminothiazol-4-yl)- alpha -(Z)-methoxyiminoacetamido]-3-[(1-methyl-1-pyrrolidinio)methyl]-3-ceph em-4-carboxylate are stable even at elevated temperatures. The crystalline sulfuric acid addition salt is made by forming an admixture of (a) at least one molar equivalent of sulfuric acid with (b) zwitterion in an amount so as to be present in the admixture at a concentration of greater than 25 milligrams/ml, causing crystallization, separating the crystals, washing and drying. The crystalline monohydrochloride, dihydrochloride, and orthophosphate salts are prepared by dissolving the zwitterion in the appropriate amount of acid, causing crystallization by adding acetone and isolating the crystals. Physical admixtures of the salts with certain bases in proportions to give a pH ranging from about 3.5 to about 7 on dilution with water provide injectable compositions on dilution.
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