CANCEKR of the bladder epithelium of the mouse may be induced by systemic or local administration of chemical carcinogens. Epithelial hyperplasia has been implicated as a relevant factor in the induction of cancer of the bladder of mice given chemicals by mouth (Clayson, Lawson, Santana and Bonser, 1965). The technique of bladder implantation has been used to test chemicals for local carcinogenic action (Jull, 1951; Allen, Boyland, Dukes, Horning and Watson, 1957). The chemical under investigation is compressed into a pellet with an " inert " vehicle and implanted surgically into the lumen of the bladder. It has been found that a wide variety of vehicles give a small to moderate incidence of carcinomas of the epithelium (3 to 17 per cent) when implanted without an added chemical (Bryan and Springberg, 1966). This may be due to the inclusion of unknown carcinogens in the pellet or because, inter alia, the pellet, by acting as a nonspecific foreign body, induces hyperplasia in the epithelium.