Human T cell leukemia virus type I (HTLV-I) or its transcriptional transactivator, Tax1, was introduced into a human osteosarcoma cell line, HOS, and a Moloney murine sarcoma virus-positive HOS cell line, S+L-HOS. These HTLV-I- or Tax1-expressing cells were injected subcutaneously into nude mice to investigate the effects of HTLV-I on their tumorigenicities. HOS cells did not form any tumors even in the presence of HTLV-I or Tax1. S+L-HOS cells did form small tumors in two-thirds of nude mice. Infection of S+L-HOS cells with HTLV-I, or transduction of Tax1 into S+L-HOS cells markedly facilitated the tumor formation, and the tumor-bearing mice showed marked splenomegaly and neutrophilia. Elevated levels of granulocyte colony-stimulating factor (G-CSF) were detected in sera of these mice and also in the culture supernatants of Tax1-expressing human cells, suggesting that G-CSF in the mouse sera was produced by the human cells. In sera of some mice with splenomegaly and neutrophilia, high levels of murine granulocyte-macrophage colony-stimulating factor (mGM-CSF) were observed, suggesting that Tax1 produced by human cells induced mouse cells to produce mGM-CSF. Only S+L-HOS cell lines expressing Tax1 showed high tumorigenicity in nude mice. Thus, this system will be a useful model of tumor formation, splenomegaly and neutrophilia dependent on Tax1.