An actinomycete, strain No. T-1124, was found to produce new macrolide antibiotics, juvenimicins. Based on the results of taxonomic studies, the strain was considered to be a new variety of Micromonospora chalcea and the name Micromonospora chalcea var. izumensis is proposed. This strain also produced everninomicin. The production of juvenimicins was stimulated by addition of ferrous sulfate and magnesium sulfate in the fermentation medium. Among juvenimicins, juvenimicin A₃ exhibited the most potent antimicrobial activities against gram-positive bacteria and furthermore was active against gram-negative bacteria.