erythromycin n an antibiotic (trade name Erythrocin or E-Mycin or Ethril or Ilosone or Pediamycin) obtained from the actinomycete Streptomyces erythreus; effective against many Gram-positive bacteria and some Gram-negative Usage(s) He notes that a survey of 10 cities conducted three years ago revealed that in Shenyang, all pneumonia cases exhibited resistance to erythromycin. Repeated doses of the antibiotics tetracycline or erythromycin over a period of one week are required to knock out the bug. That's when antibiotics like penicillin or erythromycin come in handy. Synonym(s) Erythrocin E-Mycin Ethril Ilosone Pediamycin