Spontaneous pulmonary tumors (PT3 and PT4) originating in A strain of mice were serially transplanted and their hosts were examined biologically and morphologically.Hyperestrogenism in the hosts was observed by vaginal smear method during some period of serial transplantation, and changes of the visceral organs related to the hyperestrogenism (atrophy of the male sex organs, feminization of the salivary glands and kidneys of males) were observed in the host animals.Morphological findings of the tumors changed from tubular adenocarcinoma to anaplastic carcinoma with increase of transfer generations, and also the survival period of mice was shortened. Hyperestrogenism was observed when the tumors were partially undifferentiated histologically.Extraction of gonadotropin from the tumors was performed with negative result. The mechanism of hyperestrogenism might be a production of estrogen by the tumors.