Abstract: Maidimeisu is a macrolide antibiotic produced by Streptomyces sp. 1748 isolated from Chinese soil. A study of its chernical structure showed that it is identical to mydecamycin reported in Japan. The antimicrobial activities of maidimeisu were studied in vitro and in vivo.Maidimeisu exhibited strong antimicrobial activity against gram-positive bacteria.The minimal inhibitory concentration (MIC)of maidimeisu ranged from 0.19 to 3.125 μg/ml.Contact of Staphylococcus aureus 902—56, a clinical isolate, with subinhibitory concentration of maidimeisu did not induce resistance to the same antibiotic. Hence, contrary to erythromycin, maidimeisu belongs to the non-inductive type of macrolides.Treatment of mice with staphylococcal, streptococcal and pnemococcal infections showed that maidimeisu is similar to the Japanese medecamycin.