A method for observing a cellular response to a drug, said method comprising: a) providing a population of test cells exposed to said drug in an observation chamber and performing live cell interferometry using a Michelson-type interference microscope to determine measurements proportional to the mass of each of a plurality of test cells comprising said population of test cells, wherein said live cell interferometry comprises measuring the fractional phase shift between a test beam of light propagating through the each cell of said plurality of test cells in said observation chamber and a reference beam of light passing through a reference chamber and integrating said fractional phase shift over the area of each of said plurality of test cells to provide said measurements; b) providing a population of control cells not exposed to said drug and performing live cell interferometry using a Michelson-type interference microscope to determine a measurement proportional to the mass of each of a plurality of control cells comprising said population of control cells in an observation chamber, wherein said live cell interferometry comprises measuring the fractional phase shift between a test beam of light propagating through the each cell of said plurality of control cells in said observation chamber and a reference beam of light passing through a reference chamber and integrating said fractional phase shift over the area of each of said plurality of control cells to provide said measurements; and c) comparing the measurements determined in a) with the measurements determined in b) where the differences in mass measurements in a) and b) indicates the response of each cell comprising said plurality of test cells in response to said drug. a) providing a population of test cells exposed to said drug in an observation chamber and performing live cell interferometry using a Michelson-type interference microscope to determine measurements proportional to the mass of each of a plurality of test cells comprising said population of test cells, wherein said live cell interferometry comprises measuring the fractional phase shift between a test beam of light propagating through the each cell of said plurality of test cells in said observation chamber and a reference beam of light passing through a reference chamber and integrating said fractional phase shift over the area of each of said plurality of test cells to provide said measurements; b) providing a population of control cells not exposed to said drug and performing live cell interferometry using a Michelson-type interference microscope to determine a measurement proportional to the mass of each of a plurality of control cells comprising said population of control cells in an observation chamber, wherein said live cell interferometry comprises measuring the fractional phase shift between a test beam of light propagating through the each cell of said plurality of control cells in said observation chamber and a reference beam of light passing through a reference chamber and integrating said fractional phase shift over the area of each of said plurality of control cells to provide said measurements; and c) comparing the measurements determined in a) with the measurements determined in b) where the differences in mass measurements in a) and b) indicates the response of each cell comprising said plurality of test cells in response to said drug. <EOS>