The invention relates to a porous material for body fluid treatment for promoting lymphocyte proliferation in lymphocyte culture which contains a high-molecular compound having an angle of contact with water within the range of 40 to 98°, and a porous material for body fluid treatment which comprises activated carbon; and also relates to a treatment device wherein the porous material is used; a method for proliferating lymphocytes; a method for producing mammalian lymphocytes; a method for producing a pharmaceutical composition; an additive body fluid to be added to a culture medium on the occasion of lymphocyte culture; a method for treating a disease against which a therapeutic effect is produced by returning extracorporeally activated mammalian lymphocytes into the body; and a method of manufacturing the porous materials for body fluid treatment for promoting the lymphocyte proliferation in lymphocyte culture.