An animal model of metastasis with features similar to those of lung cancer metastasis in humans is required for an understanding of the cellular and molecular mechanisms of human lung cancer metastasis. Ma-44 cell lines derived from human squamous cell lung cancer were percutaneously injected (2-3 x 10(4)) into the left lung of SCID mice. After orthotopic implantation, Ma-44 cell lines formed tumors in the left lung at a high rate (17/25, 68%), and many of those metastasized to mediastinal lymph nodes (13/17, 76%) by the 14th day, but not to other organs. After the ectopic implantation, the Ma-44 cell line inoculated subcutaneously (2-3 x 10(5)) formed a tumor at the inoculation site by the 28th day (all mice), but did not metastasize to any organs. The Ma-44 cell line inoculated intravenously (2-3 x 10(5)) formed metastases in the lungs (37/50, 74%), and these pulmonary metastases metastasized to the mediastinal lymph nodes at low rate (3/37, 8%) by the 14th day. The orthotopic sites of implantation are critical for the metastatic ability of transplanted tumors in SCID mice. Since non-small cell lung cancer (NSCLC) grows at the primary site in humans, lymphogenous metastasis occurs frequently, and blood-born metastasis occurs at moderate rate, our orthotopic SCID mice model was similar to the metastatic behavior of NSCLC in humans. Thus, this model may be useful for elucidating the mechanism of lymphogenous metastasis in human lung cancer, and testing the anti-metastatic efficacy of therapeutic agents in vivo.