By means of microspectrophotometry the author has observed the penetration of the dye qunantitatively on cells stained supravitally with Nile blue. For the quantitative estimation of dye penetrated into the cell the dye in cells was fixed at a certain period of supravital staining by the method devised by Seno, Haba et al, and the dye concentration has been estimated by microspectrophotometry at the wave length of 6000A. Observation revealed that the quantity of penetrated dye in a bone marrow cell at pH 7.0 in one minute was 19.4 x 10(-11) mg in average. The value was found to decrease with the decreased pH of the staining media reaching nearly zero at PH 3.2. In a more acidc media the penetration of the dye increased again but supposed by the destruction of the cell wall. With this method the permeability of dye to one cell in a certain period of time can be estimated exactly in mg. in the range of pH where the destruction of the cell membrane does not occur.