A method for lymphocyte purification is described. This is a closed system in which lightly heparinized blood is sedimented with Plasmagel in a plastic blood bag. The plasma layer is transferred to a second bag containing nylon powder and clotting is induced by agitation. Platelets, polymorphonuclear granulocytes, and monocytes are removed and trapped in the clot, together with the nylon powder. Lymphocyte suspension of 96 per cent purity and 61 per cent overall yield are produced. Viability of the lymphocytes is excellent and they are shown to be suitable for tissue culture. Comparison with other methods for lymphocyte isolation show this method to give higher yields and to be simpler in performance. It is particularly adaptable for large volumes of blood.