RECENTLY, Gillman et al.1 and Simpson2 reported that trypan blue injected subcutaneously into rats at weekly or bi-weekly intervals induced tumour (reticulum cell sarcoma) of the liver. Trypan blue is a sulphonated dis-azo dye and somewhat resembles in chemical structure dimethylaminoazobenzene, which is a potent carcinogen for the liver of rats.