A simple spectrophotometric method is proposed for the determination of cephalexin. The method involves acetylation of cephalexin with acetic anhydride in aqueous solution at pH 11.5 to yield alpha-acetamidocephalexin and subsequent measurement at 335 nm of alpha-acetamidocephalexin mercuric mercaptide. The method characterizes a newly developed, sensitive procedure for the determination of cephalexin in different pharmaceutical preparations. The effect of several reaction conditions were investigated. Beer's law was obeyed over the concentration range 30-340 mug/ml. The results compare favourably with those obtained by the official B.P. 1980 method.