SYNOPSISMouse bone‐marrow cells have been treated with a 1:20,000 concentration of acridine‐orange and mounted on slides coated with nutrient agar. Under these conditions cells spread themselves rapidly, and active movements can be observed within a few minutes. When viewed with blue‐violet light a variety of marrow cells can be distinguished. The nuclei fluoresce green and the cytoplasmic granules fluoresce red; this cytoplasmic granulation is similar to that observed with phase‐contrast microscopy. Photographs were obtained using Ektachrome high‐speed colour film which permitted greatly reduced exposure times. Black and white bromide prints made from colour transparencies could be made without difficulty. These have the advantage of being free f