The rodenticide ANTU (alpha-naphthylthiourea) was used in the United Kingdom mainly in the late 1940s and early 1950s. The product then contained up to 0.2% of beta-naphthylamine as an impurity, and it was finally withdrawn in 1967 as a suspected carcinogen. Fourteen cases of urothelial tumours among rodent operatives exposed to ANTU are reported: in one district four out of 27 staff were affected, and in another area two out of 10. These cases strongly suggest that the early ANTU manufactured in the United Kingdom posed a cancer hazard to users. ANTU is still made or used in various countries, though the current product may be relatively pure and no longer contaminated by beta-naphthylamine. Recent laboratory evidence shows that even pure ANTU is mutagenic in the Ames test, and the safety of this rodenticide may need review.