Metabolites of the cephapirin ß‐lactam antibiotic have not previously been reported in bovine milk. The principal metabolite was tentatatively identified as desacetylcephapirin by liquid chromatography with UV‐VIS photodiode array (LC/UV‐VIS PDA), and liquid‐chromatography‐mass‐spectrometric (LC‐MS) detection. Synthetic desacetylcephapirin was prepared by incubation of cephapirin in bovine milk and serum at 37d̀C. Also, a method for determining cephapirin in bovine milk and serum was developed. The detection limits for cephapirin and desacetylcephapirin were estimated to be 10 and 50 ug/kg, respectively, for LC/UV‐VIS PDA, and 100 and 50