Studied is the effect of high temperature, light, moisture, the type of the solvent and the reaction in the medium on the chemical stability and biological activity of the tylosine base, tylosine tartarate and tylosine phosphate, produced by the "PHARMACHIM" State Economic Corporation. The evaluation of the effect is carried out by using the spectrophotometric (in the UV-region) and microbiological methods (diffusion in agar with Sarcina lutea A. T. C. C. 7341 as a test microorganism). It is found that tylosine is stable thermally both as substance and as solution, yet it loses a large part of its activity when the solutions are exposed to ultraviolet or direct sun light. This calls the solutions of the antibiotics in the course of their preparation, storage and application to be protected from the direct effect of light and that they should be packed in light-proof material. Depending on the air moisture (72--98 per cent), the tylosine base absorbs water from 4 to 11 per cent, the tartarate--from 9 to 37.5 per cent, the phosphate--from 8 to 28.5 per cent, the tylosine tartarate (water-soluble form)--from 7 to 32 per cent, and the tylosine phosphate (granules)--from 12 to 39 per cent of their weights. This requires their storage to be limited to dry, airy premises or they should be hermetically packed. Water, ethanol, methanol, phosphate buffers with pH 6--10.0, dimethylformamide and dimethylsulphoxide do not affect the activity of tylosine, but the last two solvents inhibit the diffusion of the antibiotic in agar and are unsuitable for biological tests. The tylosine base, manufactured by the "Elanco" Company, manifests similar properties in relation to thermal stability, light sensitivity, hygroscopicity and diffusions in agar.