Phenolphthalein (PH) was commonly used as a laxative for most of the 20th cent. The use of PH in laxatives has decreased since 1997 when the FDA proposed to withdraw its classification as an OTC drug. PH has been previously evaluated in carcinogenecity studies in 1996. The major route of human exposure to PH is via ingestion, dermal contact, & inhalation of contaminated air originating from process units manufacturing the compound. In this study, the carcinogenic effects of PH were studied in the haploinsufficient p16Ink4a/p19Arf mouse model as an ongoing goal to seek model systems for toxicology & carcinogenesis studies, especially those that can provide mechanistic info. relative to understanding an agent¿s model of action. Illustrations.