A 14-d study was undertaken to test the acute toxicity of a new preparation Vetom 21.77 based on the predacious fungus Duddingtonia flagrans. A total of 40 healthy 5-day-old broiler chickens (Hubbard F15, 100 ± 5 g), that had previously gone through a required 5-days adaptation to the environment, were orally dosed with the drug for 5 consecutive days at different doses, after which their health status was assessed daily up to the end of the experiment. According to the results, no substantial changes in the physiological state of the chickens were detected during the experiment. Internal organs weighing revealed no statistically significant differences between the groups, though weight coefficient values of internal organs of treated chickens slightly exceeded those of the control group. Some haematological parameters were significantly higher in the treatment group, without going beyond reference ranges. All chickens used in the experiment survived the study. The preparation has not produced any toxic effect even at a higher dose (4000 µL/kg bw/day). It is concluded that Vetom 21.77 pertains to preparations of IV toxicity class.