Neutrophils anl lymphocytes isolated from the peripheral blood of patients with purulent infection were distributed into different subpopulations with the aid of rosette-formation tests. These subpopulations were mixed with autologous neutrophils at a ratio of 1: 100 and cultivated for 18 hours. Then erythrocytic diagnosticums were used to determine the ability of neutrophils to phagocytosis via receptors for Fc-fragments of IgM and IgG and for C', as well as that to phagocytosis without the receptors. The joint lymphocyte population did not essentially affect the percentage of different types of phagocytes. The joint population of T lymphocytes and active T lymphocytes had a powerful stimulant effect while T lymphocytes with receptors for rabbit red blood cells and a mixture of B lymphocytes with 0 cells decreased these indices. B lymphocytes with receptors for mouse red blood cells had an inhibitory action with initially high indices and a stimulant action with low ones. The test system applied enables one to differentiate between the helper and depressor activities of lymphocytes in the lymphocyte-phagocyte system, which plays an important role in the infectious process.