Lymphocyte proliferation rate was determined by ³H-thymidine incorporation assay after stimulation of peripheral blood mononuclear cells (PBMC) from BLV and BIV-infected cattle with mitogens: concanavalin A, phytohaemagglutitin, and pokeweed mitogen. BLV infection were associated with significant decrease in lymphocyte proliferation, as compared to uninfected animals. Co-infection with BIV strongly increased antimitotic influence of BLV, mainly in PL-animals. These results demonstrated that a function impairement of lymphocytes can be observed in the course of BLV and/or BLV/BIV infection.