Abstract. Studies were conducted to determine the comparative effects of tocopherols and tocotrienols on preneoplastic (CL-S1), neoplastic (-SA), and highly malignant (+SA) mouse mammary epithelial cell growth and viability in vitro. Over a 5-day culture period, treatment with 0–120 μMα- and γ-tocopherol had no effect on cell proliferation, whereas growth was inhibited 50% (IC50) as compared with controls by treatment with the following: 13, 7, and 6 μM tocotrienol-rich-fraction of palm oil (TRF); 55, 47, and 23 μMδ-tocopherol; 12, 7, and 5 μMα-tocotrienol; 8, 5, and 4 μMγ-tocotrienol; or 7, 4, and 3 μMδ-tocotrienol in CL-S1, -SA and +SA cells, respectively. Acute 24-hr exposure to 0–250 μMα- or γ-tocopherol (CL-S1, -SA, and +SA) or 0–250 μMδ-tocopherol (CL-S1) had no effect on cell viability, whereas cell viability was reduced 50% (LD50) as compared with controls by treatment with 166 or 125 μMδ-tocopherol in -SA and +SA cells, respectively. Additional LD50 doses were determined as the following: 50, 43, and 38 μM TRF; 27, 28, and 23 μMα-tocotrienol; 19, 17, and 14 μMγ-tocotrienol; or 16, 15, or 12 μMδ-tocotrienol in CL-S1, -SA, and +SA cells, respectively. Treatment-induced cell death resulted from activation of apoptosis, as indicated by DNA fragmentation. Results also showed that CL-S1, -SA, and +SA cells preferentially accumulate tocotrienols as compared with tocopherols, and this may partially explain why tocotrienols display greater biopotency than tocopherols. These data also showed that highly malignant +SA cells were the most sensitive, whereas the preneoplastic CL-S1 cells were the least sensitive to the antiproliferative and apoptotic effects of tocotrienols, and suggest that tocotrienols may have potential health benefits in preventing and/or reducing the risk of breast cancer in women.