A well-demarcated, raised, hot red area spreading fast in a lymphoedematous arm after axillary dissection is erysipelas, caused mainly by group A streptococcus. It is treated like cellulitis, and penicillin G heads the antibiotic list. Macrolides (erythromycin) and fluoroquinolones are listed only as alternatives, and metronidazole (anaerobic cover) has no role.