Passage of human peripheral blood leucocytes through Sephadex G-10 columns results in a complete depletion of monocytes. Microscopic examination and ingestion of latex particles failed to reveal any monocytes in the column-passed cells. The study of surface markers shows a slight enrichment of T-cells and a decrease of B-cells that is more pronounced in the IgG- and IgA-bearing lymphocytes. No changes were seen in the Fc receptor-positive cells. Con A-induced activation was significantly affected. Pokeweed mitogen activation was reduced 50% at optimal doses of mitogen whereas the response was unaffected at suboptimal doses. Mixed leucocyte reactions (MLR) were greatly depressed when monocytes were removed from either the responder or the stimulator cells, indicating an important role of monocytes both in the elicitation of the response and as auxiliary cells in the mixed leucocyte culture.