Directed biosynthesis of avermectins.

Avermectin homologs are produced by Streptomyces avermitilis when externally supplied with sodium 2-methylpentanoate and sodium 2-methylhexanoate. The homologs carry 2-pentyl and 2-hexyl groups, respectively, at C-25 of the aglycone moiety as opposed to the 2-butyl group of "a" components and the isopropyl group of "b" components of natural avermectins. The new homologs designated as avermectin "c" and "d" components, respectively, possess potent anthelmintic and insecticidal activity.