N(6)-Isopentenyladenosine (iPA) is a member of the cytokinins, a family of plant hormones that regulate plant cell growth and differentiation. iPA is present in mammalian cells in a free form, as a mononucleotide in the cytoplasm, or in a tRNA-bound form. Its antiproliferative activity against cancer cells lines has been reported by several authors. We show that iPA inhibits the growth of human breast cancer MCF-7 cells in a dose-dependent manner with a concentration of 12.2 μM, which causes 50% inhibition of cell viability. The cell cycle analysis by flow cytometry showed that iPA-induced growth arrest could be associated to apoptosis. The interaction of DNA with iPA was studied in aqueous solution at physiological conditions by ultraviolet-vis spectroscopy. A binding constant K(iPA-DNA) = 4.4 × 10(3) M(-1) was calculated and this value, together with the shift of absorbance, suggests iPA-DNA interaction at DNA surface.