Since l'-demethylclindamycin is a potent antibacterial agent, its production by microbial transformation of clindamycin was attempted. l'-Demethylclindamycin was produced by N-demethylation when clindamycin was added to fermentations of Streptomyces punipalus. In contrast, addition of clindamycin to fermentations of Streptomyces armentosus resulted primarily in the production of clindamycin sulfoxide. Several other species of streptomycetes as well as several species of fungi were also found to convert clindamycin to either l'-demethylclindamycin, clindamycin sulfoxide, or a mixture of both compounds. The present paper discusses the results of these studies.