We report here that y interferon produced in animal cell culture injected intraperitoneally, efficiently protects mice against Cow£iria ruminantium infection. None of the other cytokines tested exerted any effect. Neutralizing antibodies against the cytokines (anti IL6, anti TNF, anti yIFN), did not modify the course of the disease, indicating that these cytokines do not play any crucial role in the pathology. The results conceming the protective effect of interferon pave the way towards the establishment of a rational selection method for protective antigens.