In the present article, we describe a study of antitumor activity in breast cell lines using silver nanoparticles (Ag NPs) synthesized by a microbiological method. These Ag NPs were tested for their antitumor activity against MCF7 and T47D cancer cells and MCF10-A normal breast cell line. We analyzed cell viability, apoptosis induction, and endocytosis activity of those cell lines and we observed that the effects of the biosynthesized Ag NPs were directly related with the endocytosis activity. Moreover, Ag NPs had higher inhibition efficacy in tumor lines than in normal lines of breast cells, which is due to the higher endocytic activity of tumor cells compared to normal cells. In this way, we demonstrate that biosynthesized Ag NPs can be an alternative for the treatment of tumors.