Method for obtaining an anti-tumor substance from even-toe hoofed mammals (artio-dactylous animals) having leucosis, and according to a first alternative, the substance is obtained from the lipid-free blood plasma fraction of the animal, and the blood is taken from a pregnant female donor animal being in the 2nd or 3rd period of pregnancy up to at most the beginning of the first week preceding delivery. According to a second alternative the substance is obtained from the colostrum of the female donor animal, being preferably cow or sheep.