A compact, scanable, optical monochromator has been installed in the fluorescence detection channel of a FACS II instrument. By scanning the wavelength of light transmitted by this device, a fluorescence emission spectrum can be generated either averaged for an entire sample or for a subgroup of cells selected by low angle light scatter. Several applications are described including optimization of conditions of detection for the common or uncommon fluorescent stains and preliminary data from studies of the natural autofluorescence of peripheral blood granulocytes.