The synthesis, antimicrobial activity, and binding to ribosomes of leucomycin and leucomycin derivatives are described. In general, the binding of the leucomycins and the leucomycin derivatives to ribosomes correlated with their antimicrobial activity. Some 2'-O-acyl derivatives apparently underwent gradual hydrolysis during antimicrobial assays, for their binding to ribosomes was poor compared to their relatively good antimicrobial activies. Correlation between antimicrobial activity and binding to ribosomes, their molecular site of action, provides some insight into the nature of the active molecular moieties.