A number of patients have been seen with generalized histamine reactions following the use of a hair bleach. Similar urticarial and dermatitic reactions were seen on the hands of hairdressers handling this bleach preparation. The responsible agent was found to be ammonium persulfate. Evidence is given that this compound releases histamine from the skin in susceptible persons. It is a weak histamine liberator. Passive transfer studies were negative. The mechanism of action could be either direct histamine liberation or antibody-mediated.