Ponceau 3R is an azo dye that once was used as a red food colorant. Ponceau 3R is carcinogenic in rats following its oral administration, producing liver-cell tumours. It also produced bladder tumours in mice following its implantation in the urinary bladder. The oral study in mice was considered inadequate for evaluation. Ponceau 3R was reduced in vitro by Fusobacterium sp. 2, a human intestinal anaerobe, to a product which is mutagenic when metabolically activated by liver S9 preparations in the Salmonella/mammalian-microsome mutagenicity test. This mutagenic metabolite has been identified as 2,4,5-trimethylaniline.