Macrolides fall into 3 classes, depending on the size of the lactone ring.

• Erythromycin and the closely related oleandomycin and troleandomycin belong to the 14-membered ring group.
• Azithromycin and gamithromycin are 15-ring members, a subclass referred to as azalides.
• Of the 16-membered ring group, spiramycin, josamycin, tylosin, and tilmicosin (synthesized from tylosin), are used clinically.
• Tulathromycin contains 3 amine rings and is classified as a triamilide.