Metastasis of metastases.

Metastasis of metastases was demonstrated experimentally. Mice whose primary tumors were amputated after the development of pulmonary metastases were placed into parabiosis with normal syngeneic partners. Metastases were demonstrated in the nontumor-bearing partners despite their development of passive immunity. Immunizing the nontumor-bearing partner prior to joining into parabiosis was not protective against the formation of metastasis.