The absence of studies for the impact of aflatoxin B1 (AFB1) on natural immunity in ducks, motivated us to investigate its influence on the concentrations of the blood serum lysozyme and alternative pathway of complement activation among Moulard ducks. For the purpose of the study two separate experiments were conducted. The first one was among 40 duck babies divided into four groups: control group (not treated); – fodder was not supplemented with aflatoxin B1; group two- fodder + 0.5 mg/kg aflatoxin B1; group three – fodder + 0.8 mg/kg aflatoxin B1 and group four – fodder + 0.5 mg/kg aflatoxin B1 + 2 g/kg fodder Mycotox NG. The second experiment was also conducted by the same way but the blood was bled on the 50^th day. The obtained results showed that the period of treatment has significant effect to the blood serum lysozyme concentrations. Lysozyme concentration in group second decreased from 4.42 to 2.16 mg/L and in group three from 3.92 to 2.70 mg/L. Results showed influence of the toxin also on the alternative pathway of complement activation. When applied in low doses and shorter periods it has immune stimulation effect, while applied in higher doses and longer periods it suppresses the alternative pathway of complement activation (APCA). Complement activity was decreased from 727.99 CH50 to 625.91 CH50 in second group and from 816.74 CH50 to 601.61 CH50 in group three. Mycotox NG has immune stimulation effect when applied for 15 days but in all other experimental groups it did not show such effect.

Key words: Aflatoxin B1, Complement system, Lysozyme, Mycotoxin NG