the invention features methods for measuring the total amounts of g protein - bound gdp and gtp in mammalian cells and tissue . the method of the invention is advantageous over conventional methods in that the cells need not be exposed to phosphate - free growth conditions , which can adversely affect the accuracy of the gtp determination . moreover , the method does not require exposure of the cells to radiolabelled phosphate in the growth medium . standardized amounts of g protein - bound gdp and gtp are determined relative to non - nuclear cellular protein content or cellular dna content , allowing comparison of results across different cell types . the method is particularly applicable to the detection of ras protein bound gtp and gdp . the determination of ras - bound gtp levels by the method of the invention provides a sensitive and accurate means of evaluating the role of ras activation in cellular processes , such as cell proliferative disease , hormonal secretion , cellular differentiation , programmed cell death , cell cycle regulation , and protein trafficking . treatment of abnormal processes can then more accurately targeted to the alteration of g protein activation .