A macrolide antibiotic produced by Streptomyces hygroscopicus var. azalomyceticus, azalomycin-B (I), which is activity against gram-positive bacteria and Trichomonas vaginalis, was investigated as a preliminary study for the structural elucidation. A molecular formula of I was tentatively established as C₅₆H₉₂O₁₉. It was also found that an α, β, γ, δ-unsaturated ester function (X) existed as the chromophor part of the molecule.