Breast cancer is the most prevalent type of cancer around the world. It has been studied extensively thanks to several immortal cell-lines which have provided accurate and cost-effective models. Their mechanical behavior has been described recently thanks to the new techniques available and a rising interest in biomechanics. In the present study, the stiffness of three cell-lines (two cancerous, MCF-7 and MDA-MB-231, and a healthy one, MCF-10A) have been studied using a digital holographic microscope and osmotic shocks. The latter were achieved by changing the media of the cells from lower to higher concentrations of NaCl. To study their stiffness, the bulk modulus of these three cell lines was calculated by quantifying their deformation using the digital holographic microscope.