In the course of our microbial transformation study on erythromycin derivatives, Streptomyces hygroscopicus ATCC 31080, which produces a poly ether antibiotic carriomycin, was found to transform erythromycin derivatives to their inactivated derivatives. The structures of inactivated derivatives prepared by enzyme reaction using the cell extract, UDP-glucose (or UDP-galactose) and Mg²⁺ (or Mn²⁺) were elucidated on the basis of analysis of their spectral data to be the compounds glycosylated at C-2' of a desosamine moiety, indicating that the enzyme is a macrolide glycosyl transferase (MGT). The MGT activity of cell extract from S. antibiotieus ATCC 11891, a producing organism of oleandomycin, could be distinguished from that of ATCC 31080, based on the ability to glycosylate tylosin. We examined 32 actinomycete strains producing such polyketides as macrolide and polyether antibiotics, and found that 15 strains of Streptomyces have macrolide glycosyl transferase activity. It suggests that the MGTs have been distributed among at least polyketide producing Streptomyces strains.