The response to phytohemagglutinin of peripheral blood lymphocytes was studied in 169 cancer patients. There was a significant decrease compared with control groups (normal persons and those with benign disease). By selecting cancer leukocyte samples with reactivity to phytohemagglutinin that was increased by carrageenan, a macrophage-toxic agent, and by mixing them with normal lymphocytes, we have demonstrated that the depressed phytohemagglutinin of six cancer patients' lymphoyctes was due to the presence of suppressor cells that possibly were monocytes.