The mushroom of commerce in the Western hemisphere, Agaricus bisporus, was administered orally to Swiss mice that were 6 weeks old at the start of the experiment The mushrooms were baked at 220-230 degrees C for 10 min. Subsequently, the mushrooms were fed to the animals for 3 days and were followed by a semisynthetic diet for 4 days each week, for life. The treatment induced tumors in the forestomach, glandular stomach, duodenum, and ovaries in the following incidences: 20, 12, 14 and 12% in females and 16, 20, 4 and 0% in males. In the tissues of the untreated controls, only an ovarian tumor was found in a female. Histopathologically, the neoplasms were classified as squamous cell papillomas and carcinomas of the forestomach, and adenomas and adenocarcinomas of the glandular stomach, duodenum, and ovaries. Since Agaricus bisporus is mainly eaten in baked form in the United States, the findings may carry useful implications.