Abstract: A cell line, which designated BMAC, was established from spontanously adenocarcinoma in BALB/c mice. The pathologic features are typical of mammary adenocarcinoma. The C-typc virus paticles were found within the tumor and cell line by the electron microscpc. The line has been subcultured 112 passage. The cell growth is rapid and stable. The mitoticindex reached 49‰ on day 4. The chromosome number of this line was usually pscudodiploid, the modal number of chromosome being 40～41. Homotransplant efficiency was 100 %(15/15). The transplanted tumors were similar to the primary one in both morphology and histology. It was indecated that this line still retains the primary tumor's feature and behavior.