Phenolphthalein (a triphenylmethane derivative) has been commonly used as a laxative for most of the twentieth century, but little is known about its long-term carcinogenic potential in experimental studies. In our studies, phenolphthalein administered continuously in the feed for 2 years to F344 rats at doses of 0, 12,500, 25,000, and 50,000 ppm and to C57BL/6 × CH3 F1 (hereafter called B6C3F1) mice at doses of 0, 3,000, 6,000, and 12,000 ppm caused multiple carcinogenic effects. Treatment-related neoplasms occurred in the kidney and adrenal medulla in male rats, adrenal medulla in female rats, hematopoietic system in male and female mice (histiocytic sarcomas and malignant lymphomas), and ovary of female mice. Phenolphthalein has been shown to have estrogenic and clastogenic properties. Previous studies of other estrogenic chemicals (e.g., zearalenone) in the F344 rat and B6C3F1 mouse have not shown the same spectrum of carcinogenic activity as that found with phenolphthalein, suggesting that phenolphthalein estrogenic activity alone is not responsible for the spectrum of tumors observed. It is more likely that the multiple biological properties of phenolphthalein, including its ability to form free radicals, its clastogenic activity, and its estrogenic activity, contributed to the carcinogenic effects observed. These studies show that phenolphthalein is a multisite/multispecies carcinogen. One of the sites for neoplasm that is of particular concern is the ovary, and epidemiology studies are under way to identify any potential effects of phenolphthalein exposure at this site in humans.