Objective: to evaluate the clinical efficacy of the rituximab biosimilar Acellbia® at a dose of 600 mg intravenously at a 2-week interval in patients with active rheumatoid arthritis (RA) 12 and 24 weeks after initiation of treatment.Subjects and methods. Examinations were made in 20 active seropositive RA patients who had not been previously treated with biological agents (BAs), but received two infusions of the rituximab biosimilar Acellbia® at a dose of 600 mg intravenously at a 2-week interval during stable therapy with methotrexate (MT) and glucocorticoids (GCs). The European League Against Rheumatism (EULAR) response criteria (Disease Activity Score 28 (DAS28), Clinical Disease Activity Index (CDAI), and Simplified Disease Activity Index) and the American College of Rheumatology (ACR) criteria were used to evaluate the efficiency of Acellbia® therapy. Disease remission was identified by DAS28 and 2011 ACR/EULAR criteria. The safety profile (the frequency of all reported adverse events) corresponds to the data on the safety of rituximab (MabThera®).Results and discussion. At the time of inclusion, median DAS28 was 5.6 [4.9; 6.8], SDAI – 27.1 [23.0; 39.9], and CDAI – 26.6 [22.2; 37.0]. At week 12 after initiation of Acellbia® therapy, they decreased to 4.2 [3.24; 4.75], 14.4 [8.5; 20.7], and 13.2 [7.9; 19.0] respectively, which remained at 24-week follow-up (p<0.01). At week 12, the frequencies of ACR 20%, 50%, 70% improvements were 70, 55, and 5%; at week 24, these were 75, 45, and 15%, respectively. A good or moderate EULAR response at week 24 was observed in 25 and 60% of patients, respectively. At week 24, DAS28, SDAI, and CDAI remissions were achieved by 4 (20%), 2 (10%), and 1 (5%); low disease activity – by 4 (20%), 5 (25%), and 6 (30%) patients, respectively; high disease activity as measured by SDAI and CDAI remained in 3 (15%) patients. Two patients (10%) met the 2011 ACR/EULAR remission criteria at 24 weeks.Conclusion. The rituximab biosimilar Acellbia® 600 mg used in patients with active seropositive RA is clinically effective and comparable in the safety profile as shown in investigations of the brand-name MabThera® (F. Hoffman-La Roche Ltd., Switzerland) at a low dose (500 mg), as well as the first BA.