Two cytotoxic antibiotics, designated viranamycins A and B, were isolated from the culture broth of Streptomyces sp. CH41. Their structures were elucidated as new 18-membered macrolides related to virustomycin A and concanamycin A from NMR spectral analysis. Viranamycins A and B inhibited the growth of P388 mouse leukemia and KB human squamous-cell-carcinoma cells.