Five hundred and seventy-four spontaneous harderian gland neoplasms were observed in 3,123 male and 9,024 female untreated experimental mice of various ages and of 6 inbred strains and substrains and 3 hybrid stocks. Adenomas occurred in 86 males and 458 females, and adenocarcinomas were present in 3 males and 27 females. The highest tumor incidence occurred in mice after 24 months of age; no tumors were observed in any mice prior to 6 months of age. With the exception of 1 age group, both sexes of the BALB/c mice had the highest incidence, and also this strain accounted for 463 of the neoplasms. The adenomas were classified into one of four different histologic types: papillary, cystic papillary, acinar, and cystic. The papillary type was the most frequently observed in both sexes. The cystic papillary type was the second most frequently observed and was the type most frequently associated with gross lesions. This type may represent a developmental variant of the papillary type. The incidence of the acinar type was similar in both sexes. The cystic type had the lowest incidence. All except 3 adenocarcinomas appeared to evolve from adenomas, with evolvement occurring primarily in the acinar type. Five of the adenocarcinomas either metastasized to the lung or invaded periorbital tissues. The remaining 25 were confined to the harderian gland.