The refractory state of birds to the "T (S) cruzi" is well known since long ago. In previous papers it was possible to observe in chicks, infection "in ovo", still present at the last dat of incubation. Nevertheless after hatching the chicks become refractory to the parasite. The destruction of the parasites takes place in the skin at the inoculation point. Throughout these studies the authors verified that the trypomastigotes forms disappear within an hour, after being injected directly into the blood stream. Bursectomized chicks by treatment with testosterone "in ovo" plus cyclophosphamide in the first four days of life, show also the refractory condition, but their sera lack the lytic capacity to the epimastigotes of "T. (S.) cruzi", which is peculiar to normal chicken sera. Fractionation of chicken sera in column of DEAE Sephadex-A50 led to the conclusion that the gammaglobulins are the principal responsible for these lytic capacity. The dissociation of the two phenomena shows that the lytic capacity must be attributed to a "natural antibody" - since it is eliminated or become greatly reduced by depression of the immunity bursa-dependent system. Regarding to the refractory condition, the same conclusion cannot be drawn. Perhaps it is related to cellular metabolism after the process of hatching.