Prevalence of the plasma bovine leukaemia virus blocking factor in cattle from a commercial dairy herd.

The prevalence of the plasma bovine leukaemia virus blocking factor was examined in a commercial herd. The detection of the blocking factor activity depended on the nature of the infected target cells used in the assay. Using the sensitive bovine leukaemia virus-infected target lymphocytes, the factor was found to be significantly more prevalent in infected cattle than in virus-free cattle. Furthermore, in infected cattle, no correlation was observed between the level of bovine leukaemia virus antibody and the presence of the blocking factor.