Abstract The G-1 column (Adacolumn), a novel extracorporeal adsorption device, is now available for the treatment of such chronic inflammatory diseases as ulcerative colitis and rheumatoid arthritis. G-1 column treatment sometimes results in a rapid decrease in clinical inflammatory parameters and/or has a delayed beneficial effect on disease activity. In order to identify the scientific basis for such clinical benefits, we studied rats with adjuvant arthritis induced by immunization with Mycobacterium butyricum antigen. The potential role of G-1 column treatment on the migratory properties and immunoreactivities of leukocytes was investigated. Treatment of arthritic rats for 60 min with an extracorporeal perfusion through the G-1 column led to the adsorption of a small proportion (20%) of circulating granulocytes and monocytes. However, after G-1 treatment, the migration of radiolabeled blood granulocytes and monocytes to sites of acute dermal inflammatory reactions decreased significantly, in the case of granulocytes, almost by half. The migration of granulocytes to the inflamed hindpaws of severely affected animals was diminished in the G-1 treated group. Granulocytes that have passed through the G-1 column may stay in the bloodstream because of their markedly diminished number of adhesion molecules. A slightly increased accumulation in the liver and a decreased localization in the lung was also observed. These results may be relevant to the rapid clinical anti-inflammatory effect observed in rheumatoid arthritis and possibly also in ulcerative colitis, without any pulmonary complications. In contrast, the adsorption rate by the G-1 column of T lymphocytes was very low, and their migration pattern to sites of dermal inflammatory reactions was not altered after treatment. However, the antigen (Mycobacterium purified protein derivative) reactivity of T lymphocytes in blood was almost completely abolished after G-1 column treatment of arthritic rats. This unexpected qualitative effect on T lymphocytes of G-1 treatment warrants further detailed study.