A simple, accurate and highly-sensitive spectrophotometric method for the rapid determination of ampicillin trihydrate and cloxacillin sodium in pure state, their mixtures and pharmaceutical preparations is described. The method is based on the formation of deep blue colour complexes by the reaction of Amp or Clox with BTB at 25 ºC ± 2, respectively. In basic solution containing 30% (V/v) acetone.The effect of pH, time, temperature, solvent and amount of BTB have been studied. The specificabsorptivity, Sandell sensitivity, SD, and recovery percent are evaluated. The method has been used to determine each of amp. And clox. In pure form, synthetic binary mixtures and pharmaceutical preparations containing both of them.Egypt J. Pharm. Sci., 36, 235-245.