Tylosin is an antibiotic of the macrolide class (same class as erythromycin). It is made naturally by the bacterium Streptomyces fradiae and acts to inhibit bacterial protein synthesis by inhibiting the 50S ribosome, a cellular structure only certain bacteria have and use to make internal proteins. Mammals also have ribosomes to synthesize proteins but they are of a type not influenced by macrolide antibiotics.