Induction of metastases in a guest mouse in parabiotic pair of a mouse with removed tumor and a mouse with secondary transplant is demonstrated in male hybrids (CBA×C57B1/6) F₁. The minimum dose of transplanted tumor cells inducing the growth of a secondary transplant in parabiont mice is decreased. Surgical removal of primary tumor is believed to lead to the appearance of humoral factors regulating the development of tumor process.