We have investigated if positive selection for cells expressing activation antigens, which appear on the cell surface during T-lymphocyte activation, could be used for cloning purposes. For this purpose, we used paramagnetic, monodisperse Dynabeads coated with anti-Tac monoclonal antibody, which recognizes CD25 (interleukin-2 receptor light chain). After the first 6-12 h of a primary response, depletion of Tac+ cells could largely abrogate the specific response. This indicated that the specifically responding cells were found among the Tac+ population. T-cell cloning was thus performed on Tac+ blasts positively selected after 18 h of a primary response, at day 6 of a primary response or during secondary stimulation, and gave a high percentage of specific clones. This method is thus a good alternative to established techniques.