Activation of chicken complement by the classical pathway is still debatable. Both positive and negative evidence of antibody dependent lysis has been reported. The components from C3 to C9 function since activation via the alternative pathway will lyse mammalian erythrocytes in the absence of Ig. Actually the discrepancy in the literature concerning classical activation might be explained by the fact that heterologous erythrocytes were used in most studies. On the other hand, absence of lysis of chicken erythrocytes coated with specific chicken antibodies might be due to the strong anti-complementary effect ascribed to chicken serum.