There is increasing experimental evidence suggesting that extracellular and intracellular mechanical forces and deformations have a profound influence on a wide range of cell behavior such as growth, cell division and apoptosis (programmed cell death). Cells routinely deform in response to these forces under physiological conditions during tissue growth, injuries, and in arteries during systolic and diastolic cycles. In order to explore how cells adapt to applied controlled lateral deformations, we developed a novel functionalized micro force sensor that forms localized adhesion sites with single cells . The sensor can be applied to stretch or indent cells at prescribed locations, and to measure cell force response while observing the cytoskeletal actin network using GFP technique. We carried out experiments on several single monkey kidney fibroblast cells . The cells were subjected to large stretches, 1-2 times the cell size. slowly (over an hour). To our surprise, and contrary to conventional wisdom, we found that for each cell tested, the force response was linear, reversible and repeatable with small non-linearity at initial stretch. It was expected that the force response would stiffen due to alignment of actin network along the stretching direction. This reversibility may explain high energy efficiency of biological systems. We demonstrated that actin network plays a dominant role in providing mechanical integrity and strength to living cells. Under indentation, however, cell response is dramatically different. Its force response is linear to a deformation scale comparable to the undeformed cell size, followed by plastic yielding. Upon unloading, force response drops sharply resulting in strong hysteresis. In situ visualization of actin fibers reveals the mechanism of hysteresis. During indentation, actin network remodels: it depolymerizes irreversibly at discrete locations to form well-defined circular actin agglomerates all over the cell. The distribution pattern of the agglomerates strongly correlates with the arrangement of the actin fibers of the pre-indented cell Actin agglomeration has previously been observed due to biochemical treatment, gamma-radiation, and ischemic injury, and has been identified as a precursor to cell death. We believe that this is the first evidence of actin agglomeration due to local mechanical indentation and damage. These findings shed new light on the current understanding of cell mechano-biology, particularly in the area of angiogenesis, atherosclerosis, artificial tissue growth, and wound healing.