Peripheral B-lymphocyte clonality of 274 bovine leukemia virus -infected cattle with lymphocytosis was analyzed using clonality PCR based on sequences of the variable region of the bovine immunoglobulin H chain. None of the cattle showed monoclonal proliferation, while 10, 31, and 233 showed minor-clonal, oligoclonal, and polyclonal proliferation, respectively. A total of 163 cattle were analyzable the following year, and lymphocytosis was maintained in 157, indicating persistent lymphocytosis (PL). Blymphocyte clonality of the 157 PL cattle was minor-clonal in 6 (3.8 %), oligoclonal in 8(5.1 %), and polyclonal in 143 (91.1 %). A higher rate of enzootic bovine leukosis (EBL) onset within a year was observed in PL cattle with minor-clonal (50.0 % (3/6)) and oligoclonal (25.0 % (2/8)) proliferation compared to those with polyclonal (5.6% (8/143)) proliferation.Minor-clonal and oligoclonal proliferation in PL cattle may be a prognosis factor for developing EBL.

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