It had been shown that Streptomyces avermitilis UCM Ac 2161 is a potential source of variants with the raised biosynthetical activity under NG-mutagenesis. Selection was carried out under the level of 0.01-0.1 % survival. There were selected the variants with activity 5-6 times as high as that of the initial strain. Avermectin complex of these variants, selected by HPLC method is characterized by presence of A1, A2, B1 and B2 fractions. The content of fraction B1, which is usually the main basis of all antiparasitic preparations, is 40 %.