The article focuses on a simulation of a bacterial infection of fingerlings carried out in laboratory conditions in order to determine the effectiveness of the developed probiotic. The conditionally pathogenic strains of the genera Pseudomonas and Aeromonas have acted as an infectious agent. Carp fry were used as the test material for disease modeling and studying the effectiveness of a biological product developed for the prevention and treatment of bacterial diseases of fish. The experiment involved two experimental groups and the control one. The probiotic was given to the first experimental group as a feed additive before and after infection; the probiotic was not given to the second experimental group. Species in the control group were not infected and a biological product was not given. Attempts to infect carp fry with an association of strains Pseudomonas taiwanensis , Pseudomonas aeruginosa did not produce any results: after a 2-week period no deviations in the behavior of the fish were recorded, and no fish were killed. Such a situation could be caused by low pathogenicity or by the absence of pathogenicity in museum conditionally pathogenic strains. In the next experiment there was used a more aggressive strain Aeromonas punctata known as the causative agent of fish aeromonosis. Already on the third day after infecting with the pathogen Aeromonas punctata , fish in the second experimental group were killed, and deviations were observed in the form of low mobility of fish and spinal disorders of the type of twisting, which is a consequence of developing bacteriosis - aeromonosis. In fish of the first experimental group that received the previous dose of probiotic no violations were noted, all the fry remained alive. Thus, the therapeutic effect of the developed probiotic was proved on the aeromonosis model, which was expressed in a decrease in the total fish mortality by 27%.