Abstract: An adherent, stable, continuous human breast carcinoma cell line (GI-101A) has been produced from an infiltrating ductal breast carcinoma xenograft (GI-101) which has been grown and maintained in athymic mice for the past nine years. The GI-101A cells grow with an average doubling time of about 48 to about 72 hours. The cells display antigenic determinants consistent with those of the human breast tumor xenograft (GI-101) from which it was derived. The cell line, GI-101A, when injected subcutaneously into the subaxial area of athymic animals, such as athymic mice, produces tumors that spontaneously metastasize to distant organ sites, such as the lungs and lymph nodes. The cell line and the tumors that it produces may be used as model systems for study mechanisms responsible for metastatic behavior and for testing for new as well as screening for effective new anti-cancer drug therapies.