Proliferation assay for peripheral blood mononuclear cells (PBMCs) based on incorporation of [ 3 H]-thymidine by cellular DNA, routinely used for human and mouse system, has been ineffective in the woodchuck PBMCs [97, 98] . The failure of this approach is consistent with the fact that woodchuck lymphocytes do not express the thymidine kinase gene (Menne et al., unpublished results). This obstacle had been overcome by usage of the alternative radioactively labeled nucleotide 2[ 3 H]-adenine [72] . Development of 2[ 3 H]-adenine-based proliferation assay enabled to detect the T-helper lymphocyte responses after stimulation of woodchuck PBMCs with WHV core, surface and X antigens (WHcAg, WHsAg, and WHxAg, resp.) [72, 99] . In addition, using the 2[ 3 H]-adenine-based proliferation assay in PBMCs from acutely infected animals, several T-helper epitopes within WHcAg [72] and WHsAg were identified [Menne et al., unpublished results] .