The effect of different routes of inoculation on the incubation period, as indicated by the detection of antibody and by the detection of bovine leukosis virus (BLV) in lymphocytes, were compared. None of the 12-month-old steers exposed to BLV by the oral route developed BLV infection. Intratracheal, subcutaneous and intradermal inoculations were found to be particularly effective in establishing BLV infection, which was detected three to four weeks after inoculation. In the majority of animals, serum antibody and virus were detected at the same time. One out of four in-oestrus heifers inoculated via the uterus with mixtures of BLV infected lymphocytes and semen became infected. It appears that there is an inhibitory factor in fresh semen that prevents BLV infection from becoming established. Viral antigen was detected earlier in BLV infected lymphocytes using the cocultivation method than by electron microscopy to demonstrate BLV particles in mitogen stimulated lymphocytes.