The synthesis of [14C]-labeled cefepime has been reported: The cyclization of [14C]-thiourea (I) with 4-bromo-2-(methoxyimino)-3-oxobutyric acid ethyl ester (II) in ethanol gives 2-(2-aminothiazol-4-yl)-2-(methoxyimino)acetic acid ethyl ester (III), which is hydrolyzed with NaOH in methanol to the corresponding free acid (IV). The esterification of (IV) with 1-hydroxybenzotriazole (V) by means of dicyclohexylcarbodiimide in THF affords the corresponding active ester (VI), which is condensed with 7-amino-3-(1-methylpyrrolidiniomethyl)-3-cephem-4-carboxylate (VII) by means of NaOH in THF-water and isolated as the sulfate salt (VIII). Finally, this compound is treated with triethylamine in N-methyl-2-pyrrolidone.