Kefir is produced by adding kefir grains (a mass of proteins, polysaccharides, bacteria, and yeast) to pasteurized milk; it has been shown to control several cellular types of cancer, such as Sarcoma 180 in mice, Lewis lung carcinoma, and human mammary cancer. Human T-cell lymphotropic virus type 1 (HTLV-1) is the causative agent of adult T-cell leukemia, which is a fatal disease with no effective treatment. The current study aims at investigating the effect of a cell-free fraction of kefir on HuT-102 cells, which are HTLV-1-positive malignant T-lymphocytes. Cells were incubated with different kefir concentrations: the cytotoxicity of the compound was evaluated by determining the percentage viability of cells. The effect of all the noncytotoxic concentrations of kefir cell-free fraction on the proliferation of HuT-102 cells was then assessed. The levels of transforming growth factor (TGF)-alpha mRNA upon kefir treatment were then analyzed using reverse transcriptase polymerase chain reaction. Finally, the growth inhibitory effects of kefir on cell cycle progression and/or apoptosis were assessed by flow cytometry. The maximum cytotoxicity recorded at 80 microg/microL for 48 hours was only 43%. The percent reduction in proliferation was very significant, dose and time dependent, and reached 98% upon 60-microg/microL treatment for 24 hours. Kefir cell-free fraction caused the downregulation of TGF-alpha, which is a cytokine that induces the proliferation and replication of cells. Finally, a marked increase in cell cycle distribution was noted in the pre-G1 phase. In conclusion, kefir is effective in inhibiting proliferation and inducing apoptosis of HTLV-1-positive malignant T-lymphocytes. Therefore, further in vivo investigation is highly recommended.