Noncultivated and short-term cultivated blood leukocytes of BLV-infected cattle both with and without tumorous leukosis have been investigated for BLV-p24 antigen expression and for their capacity to bind BLV-p24 antigen. In animals with persistent lymphocytosis using cell extracts of 3-6 X 10(8) non-short-term cultivated blood leukocytes, BLV-p24 antigen could be identified by means of competitive RIA. In cattle with tumorous leukosis, the antigen detection in non-short-term cultivated leukocytes may be masked by the presence of antigen-binding receptors. The capacity of p24 antigen binding is likely to be a phenotypical marker for the cells occurring in the tumour phase only.