Studies are the following preparations, produced by "PHARMACHIM": tylosine-base (Tylosine-50), tylosine tartarate (Tylosine pulvis) and tylosine phosphate (substance and granules) and their analogues: tylosine-base (Tylan-50), tylosine tartarate (Tylan soluble) and tylosine phosphate (Tylan AF 99), produced by the "Elanco" Company. The biological activity of the preparation was determined by means of the microbiological method of diffusion in agar. In the case of all experimental treatments, the preparations were compared in conformity with their biological activity expressed in IU/mg of the preparations. The minimum inhibition concentrations of the preparations were determined by the method of serial dilutions in liquid nutritive media on 19 different pathogenic strains. The antispirochetal activity of the preparations compared was tested on 130 cockerels and pullets, experimentally infected with Borrelia anserina of the Surnevo and Pamoukchii serotypes. The application of the preparations started simultaneously with the infection or 24 and 48 hrs after it and continued for days. No significant differences are found in the antibacterial and antispirochetal activities of the Bulgarian tylosine preparations and those manufactured by the "Elanko" Company. The tylosine base does not differ significantly in its antimicrobial and antispirochetal activities from the tylosine phosphate and the tylosine tartarate when equal in IU concentrations and doses are applied.