For the first time we here present data on the biocidal action of new semisynthetic derivatives of avermectin B1 that we have synthetized earlier. <...> These are the 16-membered macrocyclic lactones, the representatives of an important class of anthelmintics. <...> In 2015 S. Omura (Japan) and W. Campbell (USA) who discovered this avermectin group, were awarded the Nobel Prize in physiology and medicine. <...> In our study the oligochaetes Tubificidal tubifex were used as a test-object. <...> The original chemicals and synthetized derivatives tested were avermectin B1 (abamectin), ivermectin, monosaccharide analogues of abamectin and ivermectin, namely abamectin, ivermectin, 5-O-succinyl avermectin B1, methyl ester of 5-O-succinyl avermectin B1, ethyl ester of 5-O-succinyl avermectin B1, diethyl ester of 5,4-di-O-succinyl avermectin B1, ethyl ester of 5-Omalonyl avermectin B1, diethyl ester of 5,4-di-O-dimalonyl avermectin B1, monosaccharide hemisuccinate of avermectin B1 (5-O-succinyl-4´-dezoleandrozyl-4´-hydroxyavermectin B1), ethyl ester of 5-O-succinyl-4-O-chloroacetyl avermectin B1, 5-O-succinyl ivermectin, ethyl ester of 5-O-succinoyl ivermectin, 5,4-di-O-succinyl ivermectin, diethyl ester of 5,4-di-O-succinyl-ivermectin, ethyl ester of 5-O-malonylivermectin, diethyl ester of 5,4-di-O-dimalonyl ivermectin, monoavermectin-5-yl ester of 4-[2-(4-nitrophenyl)-2-oxoethoxy]-4-oxobutanoic acid, monoavermectin-5-yl ester of 4-[2-(4-chlorophenyl)-2-oxoethoxy]-4-oxobutanoic acid, monoavermectin-5-yl ester of 4-[(4-nitrophenyl)-methoxy]-butanoic acid, monoavermectin-5-yl ester of 4-[1-methyl-2-(4-methylphenyl)2-oxoethoxy]-4-oxobutanoic acid, monoavermectin-5-yl ester of 4-[2-(4-chlorophenyl)-1-methyl-2oxoethoxy]-4-oxobutanoic acid, monoavermectin-5-yl ester of 4-[3-chloro-1-(4-сhlorbenzoil)-propoxy]-4-oxobutanoic acid, monoavermectin-5-yl ester of 4-{2-[(4-methylphenyl)-amino]-2-oxoethoxy}-4-oxobutanoic acid and monoavermectin-5-yl ester of 4-{2-[(4-bromophenyl)-amino]-2-oxoethoxy}-4-oxobutanoic acid. <...> The acute toxicity (LD50) of the most effective ones, 5-O-succinyl avermectin B1, 5-O-ethylsuccinyl avermectin B1 and 5,4-di-О-ethylsuccinyl avermectin B1, for intraperitoneally challenged white mice was 37.85; 41.37 and 45.82 mg/kg, respectively. <...> We also used membrane preparations of rat brain as in vitro model for screening and studying activity of natural and semi-synthetic avermectins. <...> A radioligand [G-3H]SR 95531 binding assay of avermectin B1, ivermectin, and 5-O-succinyl avermectin B1 <...>