Consumption of the Agaricus species mushrooms has increased considerably in Japan as the Japanese have become accustomed to Western cooking. The Agaricus species mushroom contains hydrazine derivatives known as Agaritine. Bladder implantation was performed to test the carcinogenic potential of the Agaricus species mushroom which contains large quantities of Agaritine. The results are summarized as follows: 1) Agaritine was detected in fresh Agaricus bisporus mushrooms at the level of 228.2 micrograms/wet weight and in Shiitake (Agaricus edodes) mushrooms at 0.82 microgram/g wet weight, but was not detected in either canned Agaricus bisporus mushrooms or Matsutake (Armillaria edodes) mushrooms. 2) The observed rates of mouse urinary bladder carcinoma were 30.8% for fresh mushrooms (Agaricus bisporus), 23.5% for fresh Shiitake (Agaricus edodes), 9.8% for dry Shiitake, 50% for synthesized Agaritine and 5.4% for paraffin wax as a control. The methanol extract of fresh mushrooms (Agaricus bisporus) and synthesized Agaritine were found to be significantly carcinogenic on the mouse bladder epithelium by the bladder implantation test with a probability of less than 0.01. 3) A large quantity of Agaritine was detected in fresh mushrooms (Agaricus bisporus), but decreased after boiling the mushrooms in water at 100 degrees C for 10 min. 4) The methanol extract of fresh mushrooms (Agaricus bisporus) and synthesized Agaritine are suggested to be the agents for producing cancer in the bladder epithelium. 5) Adding heat to mushrooms containing Agaritine before cooking contributes to the prevention of any potential Agaritine hazard which may induce carcinogenic changes in the bladder epithelium.