Relevance. Excess methionine entering the body (with food, drugs, dietary supplements) is accompanied by toxic accumulation of homocysteine and adverse effects leading to the formation of pathology of some systems. Small amount of studies are devoted to the comparative effect of methionine isomers. Objective: to establish the features of the biological effect of methionine isomers used as a dietary supplement on biochemical and cytological parameters of the blood of experimental animals. Materials and methods. Experimental rats received a feed additive with an increased content of methionine in the form of DL-form (racemate DL-methionine, Belgium) or L-form (100% L-methionine, Fingres Biotech Inc, China) for 0.15 g / 100 g of animal weight. In control animals, the feed did not contain additives. After 25 days, blood was taken from the animals under ether anesthesia by transcardial puncture. The following was determined: homocysteine concentration (Axis-Shield PLC (UK)); cytological analysis of white and red blood was performed (System XS-500i hematological analyzer (Japan)); for qualitative assessment, blood smears were stained with the hematological dye Leukodif-200 (Lachema, Czech Republic) and viewed under a Carl Zeiss microscope (Germany) at a magnification of × 1000. The body’s allergization index and the erythrocyte loading coefficient were calculated. Statistical analysis was performed using the Shapiro-Wilk normality analysis of data distribution, one-way ANOVA analysis of variance, multiple comparisons were performed using the Tukey test. Results and discussion. Introduction of racemic mixture of DL-methionine resulted in increase of homocysteine in blood to 28.9 ± 2.65 (p = 0.05), L-form to 79.38 ± 3.91 (p = 0.05) μmol/l (in control 8.5 ± 0.6 μmol/l). In blood of animals of group 1 increase of absolute number of granulocytic forms of leukocytes (by 1.4 times, p = 0.049) was revealed, neutrophilia was noted. When introducing L-form of methionine in animals there was sharp decrease of both absolute (by 3.7 times in comparison with control group, p = 0.01) and relative number of neutrophils, appearance of hypersegmented forms of neutrophils and immature forms of eosinophils was noted. In both groups, allergic predisposition of the organism was manifested, more pronounced in animals with the introduction of the L-form of methionine, the allergization index was increased by 49.2 ± 3.61% (p = 0.01). With the introduction of the L-form, the number of platelets decreased; ESR increased; the erythrocyte loading coefficient increased by 15.7% (p = 0.043). Conclusion. Thus, the severity of hyperhomocysteinemia, the direction of the responses of white and red blood, the coefficients reflecting the allergic disposition of the body and the functional erythrocyte load with excessive administration of methionine as a dietary supplement are determined by the form of the isomer included in its composition.