Novel avermectins produced by mutational biosynthesis.

Avermectins with a wide range of novel C-25 substituents have been prepared by feeding carboxylic acids or their biosynthetic precursors to a Streptomyces avermitilis mutant strain ATCC 53568. This organism lacks the ability to form isobutyric and S-2-methylbutyric acids from their 2-oxo acid precursors and thus is unable to produce natural avermectins unless supplied with these acids. The novel avermectins produced by mutational biosynthesis possess broad-spectrum antiparasitic activity.