The experiments reported here sho~ concomitant development of granulocytosls and enhancement of metastasis formation in C3Hf/Kam mice hearing NFSA fibrosarcoma. Both phenomena developed at approximately 2 weeks after s.c. transplantation of tumor cells, at a time when the tumor was approximately 10 mm in diameter. The number of granulocytes in the blood doubled approximately every 3"5 days, reaching about 30 times control levels shortly before the death of the mice. The magnitude of the metastasis enhancement formation by i.v. injection of tumor cells was three to four times the control value. Mice bearing NFSA had a significant increase in the number of endogenous CFUs. Plasma from NFSA-bearing mice and medium from cultured NFSA cells stimulated in vitro growth of granulocyte and macrophage colonies from normal bone marrow cell precursors, and induced granulocytosis upon i.v. injection into normal mice. This shows that NFSA tumor secretes factor(s) with potent granulopoietic activity. Injection of plasma from tumor-bearing ammals followed by i.v. injection of NFSA cells did not lead to the enhancement of metastasis, implying that granulocytosis might be rather concomitant manifestation than a causative factor of the enhancement of metastasis formation. Importance of granulocytosis as a paraneoplastic manifestation during tumor growth is discussed.