A simple, accurate, rapid and sensitive spectrophotometric method has been developed for the assay of six phenothiazine derivatives in bulk drug and their pharmaceutical preparations. The method is based on ion-pair complex reaction of phenothiazines with bromocresol green in aqueous acidic buffer. The chromogen, being extractable with chloroform, could be measured quantitatively at 420 nm. All variables were studied in order to optimize the reaction conditions. The proposed method has been successfully applied to the analysis of the bulk drugs and their dosage forms, tablets and injections. No interference was observed from common pharmaceutical adjuvants. Statistical comparison of the results with those of an official method shows excellent agreement and indicates no significant difference in precision.