FIELD: experimental medicine; microbiology. SUBSTANCE: method involves observing optical response of living microbes in osmotic pressure gradient made by hypertonic and isotonic media with respect to micro- organism cytoplasm having the same value of light refraction coefficient and hydrogen ion concentration. Sodium chloride solution is used as hypertonic medium characterized by concentration providing maximum suspension optical density for the microbe species under examination. Solution of an indifferent substance (non- electrolyte) as isotonic medium is taken in concentration close to bacterial intercellular osmotic pressure. To calculate the number of living cells, absolute and relative opto-osmotic response index for microbial suspensions are used. This way, viable bacteria contents in the preparation is determinable with consideration of their plasmolytic capability. The method gives information about absolute concentration of viable bacteria at the main stages of production of bacterial preparations. EFFECT: practically sufficient accuracy and reproducibility in determining bacterial contents in biological preparations. 4 cl, 2 tbl
