The incidence of spontaneously occurring neoplasms was observed over 10 years in a colony of germfree F344 rats (78 males and 102 females) representing 10 generations of inbredding. Leukemia was the most common neoplasm (25.6% of males, 36% of females) followed by mammary tumors (11.5% of males, 19.6% of females). Various other tumors developed in 9% of the males and 5% of the females. The overall incidence of spontaneous tumors was comparable to that reported for conventional rats of different strains. This was particularly true of leukemia for which the most data were available. However, for unknown reasons, significantly fewer solid tumors were observed in germfree than in conventional male rats.