Chemical substitutions at pharmacologically relevant sites such as C-5, C-13, C-22,23, and C-25 were examined in ivermectin, doramectin, selamectin, and a series of 11 other intermediates using a larval development assay with Haemonchus contortus. A range of activities spanning 5 orders of magnitude were manifest with small changes in the substituents to the 14 avermectins. Within this compound series, there was no major potency advantage or disadvantage to a disaccharide over a monosaccharide substituent at C-13. Ivermectin and doramectin were each fully effective at a concentration of 0.001 microg/ml, and both were similar to their respective monosaccharide homologs. Specific patterns emerged among the analogs with substituents at C-5. Analogs possessing hydroxyl groups at C-5 were superior in activity by several orders of magnitude over those with oxo substituents. Replacement of the oxo with an oxime (NOH) restored activity to some degree but did not restore it to the level of those possessing the hydroxyl substituent. Consequently, ivermectin and doramectin that possess hydroxyl moieties at C-5 were superior against H. contortus to those like selamectin that have oxime substituents. There was no advantage for analogs with a single or double bond at C-22,23 within the cyclohexyl series, and these analogs had equivalent activity as those with a single bond at C-22,23 in the sec-butyl/isopropyl series. However, there was superior activity for the analog series that possessed the combination of a double-bond at C-22,23 and a sec-butyl/isopropyl substituent at C-25. As a result, the most potent compound in this test was not any of the 3 commercialized avermectins but was a monosaccharide with a double bond at C-22,23, an hydroxyl at C-5, and a sec-butyl/isopropyl moiety at C-25.