Modern technologies for the manufacture of phage preparations provide a high degree of purification from the products of bacterial activity, endotoxins and exotoxins, products of phagolysis of bacterial cells. However, bacteriophage preparations should be tested for safety and the absence of bacterial or other toxic contaminants. Confirmation of the safety of bacteriophage drugs is the study of their toxicity administered into the stomach once or over a period of time.The aim of the study was to determine the acute and chronic toxicity of the drug based on bacteriophages "Fagomast" for the treatment of mastitis in cows.Experimental studies to determine the toxicity of the drug "Fagomast" were performed on white mice by oral administration according to current methods. Clinical trials have been conducted in accordance with the ethical principles of the European Convention for the Protection of Vertebrate Animals Used for Research and Other Scientific Purposes. The results were calculated by the Kerber method. The experiment to determine acute toxicity lasted for 14 days. During the experiment, the behavior of animals was observed, and the number of deaths in each group was recorded.In vivo toxicity assessment, namely the determination of acute toxicity with a single administration and repeated use of the drug aims to determine the potential development of bacteriophage-induced toxic effects on living organisms. Studies have shown that the acute toxicity of the drug "Fagomast" corresponds to the LD50 -5000 mg / kg live weight of animals. The drug does not have cumulative properties. The concentration of the drug does not cause irritation. It has no sensitizing properties and is non-toxic when ingested orally. Blood morphological parameters did not change significantly. According to the established signs, the working solutions of the drug "Fagomast" are classified as class IV (low-toxic) safety.