We have obtained three hybrid cell lines- FA4, FB4 and FB6 - through hybridization process between rat NMU mammary carcinoma cell line and allogeneic thymocytes. Those hybrid cell lines were inoculated with matrigel into the interscapular subcutaneous tissue in newborn Sprague-Dawley rats. About 5-14 weeks later, metastatic nodules were developed in the lungs and axillary lymph nodes among the animals, which were inoculated by FB6 or transplanted subsequently by those tumor masses. The expression of Thy 1.1 and T cell antigen was higher in FB6-12, one of the established cell lines which were gained from metastatic tumor lesions by inoculation of FB6, than in other cell lines. Also we have obtained FB6-HM subclone which was selected as a highly metastatic clone from FB6 by limiting dilution method. the evidence of hybridization between NMU and thymocytes was disclosed by the fact that FB6-HM clone showed a positive reaction against polyclonal rabbit sera raised by repeated immunization with S-D rat thymocytes. CD44 molecule, which has been known that the degree of its expression in the neoplastic epithelial cells was positively correlated with the increased metastatic potential, was also more expressed in FB6-12 than in any other cell lines. The results of this study strongly supported the hypothesis that epithelial tumor cells could get metastatic potential through hybridization with lymphoreticular lineage cells. Additionally, by establishing the metastasizing mammary carcinoma in Sprague-Dawley rat model we could perform proper experiments on the study of mechanism of hematogenous and lymphatogenous metastasis in cancer.