ABSTRACT An in vitro test on the pharmacological interaction of the antibiotic combination between florfenicol and tylosin tartrate was performed. An in vitro challenge method was used for this purpose. The results were used to calculate their interaction using the graphical representation known as ISOBOLOGRAM. The pure salts of florfenicol and tylosin tartrate were used. The agar bioassay method was used. The following criteria were used to quantify the results: A synergistic effect was defined when the actual/theoretical effect (SMEF) was greater than 1. The additive effect was defined when the SMEF was equal to 1. The area between antagonism and additive effect was defined as the zone of indifference. When studying the interaction between florfenicol and tylosin tartrate it was observed that the best SMF of this antibiotic combination was 1.45 when the concentration ratio between the antibiotics was 1 : 4 (florfenicol tylosin tartrate respectively). Consequently, it can be postulated that the combination of florfenicol with tylosin tartrate showed a synergistic effect and can be recommended in poultry and swine for the treatment of mycoplasma infections, most gram + and gram- germs and other infectious agents sensitive to the formulation.