The potential hazards of benzidine release from commercial dyes were assessed by exposing both sexes of F1 hybrid and monohybrid mice to benzidine dihydrochloride at concentrations of 0-160 parts per million for males and 0-120 parts per million for females. Dose/response effects on body weight, survival, and liver neoplasms were noted. The virtually safe dose predicted to produce less than one per million F1 female mice with a liver tumor was 0.045 parts per billion. Hence, benzidine is a very potent carcinogen. -- AATA