This study indicates that tetrazolium salts are selectively adsorbed to the mitochondria and sarcoplasmic reticulum in striated muscle of man and other animals. The adsorption is not affected by pretreatment of the section in cold acetone. Cold formol sucrose fixation enhances the definition of the stain. This selective binding indicates that considerable caution must be used in the interpretation of the staining patterns in the techniques to demonstrate the oxidative enzymes using these tetrazolium salts.