Erythromycin is a macrolide antibiotic which has an antimicrobial spectrum similar to or slightly wider than that of penicillin, and is often used for people who have an allergy to penicillins. For respiratory tract infections, it has better coverage of atypical organisms, including mycoplasma and Legionellosis. It is also used to treat outbreaks of chlamydia, syphilis, acne and gonorrhea. Structurally, this macrocyclic compound contains a 14-membered lactone ring with ten asymmetric centers and two sugars (L-cladinose and D-desoamine), making it a compound very difficult to produce via synthetic methods.

Erythromycin is produced from a strain of the actinomycete Saccharopolyspora erythraea, formerly known as Streptomyces erythraeus.