Here, we used a sheep bioassay to determine the effect of freezing colostrum to prevent the transmission of bovine leukemia virus (BLV) among neonatal calves. Leukocytes were isolated from the colostrum of a BLV-infected Holstein cow and were then either left untreated (control) or freeze-thawed. A sheep inoculated intraperitoneally with the untreated leukocytes was infected with BLV at 3 weeks after inoculation, whereas the sheep inoculated with treated leukocytes did not become infected. The uninfected sheep was inoculated again with leukocytes isolated from the colostrum of another BLV-infected Holstein cow after freezing treatment, and again it did not become infected with BLV. Finally, this sheep was inoculated with the leukocytes isolated from the colostrum of another virus-infected cow without freezing treatment, and it became infected with BLV at 4 weeks after inoculation. The results indicate that colostrum should be frozen as a useful means of inactivating the infectivity of BLV-infected lymphocytes.