One of the important methods to study the intracellular regulation of metabolism is the fluorescent spectroscopy. Fluorochrome acridine orange (AO) is widely used to investigate the nucleic acids in living as well as fixed cells. The objective of this research was to study the functional activity of the cell synthesizing apparatus with spectral fluorescent method. The cell functional (synthetic) activity is described by the parameter α representing the RNA/DNA ratio in fixed cells stained with AO. The possibilities of fluorescent cytodiagnosis have been studied to analyze the blood system cells in some diseases, under ionizing and electromagnetic irradiations, in natural and artificial hypomethabolic states as well as nervous and reproductive systems in the mollusks.