Of late many investigators have come to study blood cells with fluorescence microscope, but most of these works are carried out by supravital staining method only. Therefore, the author has attempted vital observations of bone marrow cells under the fluorescence microscope by adding fluorescent dye, acridine orange, to the medium of the simple tissue culture, the method devised in our department. For this study some fundamental studies have been conducted concerning the toxicity of dye, the selection of barrier filters, the secondary fluorescence of cells, and the influence of exciting rays on the cell growth; and obtained the following results. 1. After studying the relative growth rate, the cell density index, and the wandering velocity of neutrophils it has been found that. tissue culture is possible at low concentration, under 10(-4) of the medium with acridine orange. 2. Barrier filter, OG5 , is the most suitable one for the observation of the secondary fluorescence of the cell. 3. The concentration of the acridine orange medium at which the most distinct picture of fluorescence obtainable is at 10(-4) , and the concentration at 10(-5) is the minimum at which fluorescence can be recognized. 4. The ill effect on cells due to exciting rays may be eliminated by avoiding successive exposure to the rays. 5. The author has devised a medium for the bone marrow tissue culture, consisted of a drop of serum and a drop of physiological saline solution containing 80 γ / cc vitamin B(12) and 0.2 mg / cc acridine orange, and has designated this as the simple method of fluorochrominized bone marrow tissue culture.