Structure-activity relationships of tylosin and related compounds were evaluated in terms of their antimicrobial and ribosome-binding activities. Demycarosyl derivatives, demycarosyltylosin and 20-deoxydemycarosylrelomycin, were slightly weaker than tylosin and 20-deoxyrelomycin, respectively, both in antimicrobial activity and in affinity to ribosomes. The corresponding demycarosyl-demycinosyl derivatives had weaker antimicrobial activities despite their relatively high affinities to ribosomes. A 23-deoxy-demycarosyl-demycinosyl derivative, 20-oxo-5-O-mycaminosylprotylonolide, had a higher affinity to ribosomes than that of tylosin and was equivalent to tylosin in antimicrobial activity against Gram-positive bacteria. These results suggest that the mycinose moiety increases the ability of the molecule to enter bacterial cells. Among the derivatives tested, a 23-iodo derivative, 20-deoxy-23-iodo-5-O-mycarosyltylonolide, had the highest affinity for ribosomes as well as the highest antimicrobial activity.