We clarified the role of mold exposure in pituitary tumor induction using 53 germ-free Wistar rats exposed before birth and continually exposed thereafter, to Aspergillus versicolor alone. Rats were autopsied at 540 to 730 days. Untreated germ-free controls were autopsied on day 730. Thirty-three of the 53 monoassociated rats had pituitary tumors (62.3%), while only three of the 41 control germ-free rats had tumors (7.3%). It seems likely that this high incidence of pituitary tumor in the experimental rats is due to metabolites derived from the mold.