In this study, we investigated the effect of amine-and hydroxyl-terminated Generation 2 and 3 (G2 and G3) poly(amidoamine) (PAMAM) dendrimers on the viability and induction of apoptosis in human MCF-7 and MDA-MB-231 breast cancer cells. It was shown that amino-terminated PAMAM dendrimers were cytotoxic, with IC 50 value after 24 hr of incubation in MCF-7 and MDA-MB-231 breast cancer cells 153 ± 3 µM and 140 ± 2 µM for G2 PAMAM and 120 ± 3 µM and 99 ± 2 µM for G3 PAMAM, respectively. Experiments made with annexin V-fluorescein isothiocyanate (V-FITC) and detection of apoptosis by a fluorescent microscopy assay revealed that G2 and G3 PAMAM-NH 2 dendrimers inhibited the proliferation of MCF-7 and MDA-MB-231 malignant cells by increasing the number of apoptotic and necrotic cells. The cytotoxic and apoptotic effect of G2 and G3 PAMAM-OH dendrimers was significantly weaker.