Abstract. The aim of the present experiment was to investigate the toxic effects of aflatoxin В1 (AFB1) on production traits (weight gain, feed intake and feed conversion), antibody titre (antihaemagglutinins) after vaccination against Newcastle disease, relative weights of immunocompetent organs (thymus, spleen, bursa of Fabricius) and changes in their morphology. Also, the study aimed at testing the possibility for prevention of toxic effects of AFB1 by supplementation of poultry feed with the mycosorbent (Mycotоx NG). The experiments were conducted with five groups of ten 7-day-old Cobb broiler chickens in each. The formed groups were: group I – control, fed a standard compound feed; group II – experimental, whose feed was supplemented with 1 g/kg Mycotox NG, group III – experimental, receiving 0.5 mg/kg AFB1; group IV – experimental, receiving 0.8 mg/kg AFB1 and group V – experimental, supplemented with 0.5 mg/kg AFB1 and 1 g/kg Mycotox NG. The experiment’s duration was 42 days. Production traits were evaluated on days 21, 35 and 49 days of age. The differences in relative weights of immune organs between control and experimental groups were determined after the end of the trial. Antibody titres after vaccination against Newcastle disease were determined in blood samples collected from v. metatarsalis medialis on days 21 and 42. Lower weight gain, feed intake, increased feed conversion ratio and relative spleen weight were established in experimental groups ІІI and IV. At the same time, relative weights of the thymus and bursa of Fabricius were statistically significantly lower than those in controls. Lower antibody titres were demonstrated in groups III and IV vs untreated birds. Atrophy and degenerative changes were observed in immunocompetent organs of birds from groups ІІI and IV. The supplementation of the feed with 1 g/kg Mycotox NG resulted in statistically significant reduction of the deleterious effects of AFB1 on production traits, relative weights of immunocompetent organs and histological changes. Also, dietary supplementation of group V with mycosorbent protected birds against the inhibiting effect of AFВ1 on antibody formation against Newcastle disease.