The suppressor cells found in the spleens of mice bearing advanced tumors could be divided into two different populations by centrifugation on Hypaque-Ficoll of specific gravity 1.08. The cells in the light fraction suppressed the in vitro PFC response to SRBC and DNP-LPS. They appeared to be of the macrophage/monocyte line since they were adherent to plastic and nylon wool but were not removed by anti-BA-ϑ serum plus complement (C). They did not require DNA synthesis for activity since they were not inactivated by mitomycin C. The cells in the dense fraction suppressed only the response to SRBC. They may have been T cells since they were not adherent to plastic or nylon wool and their activity was reduced by treatment with anti-BA-ϑ serum plus C. They did require DNA synthesis since their activity was removed by treatment with mitomycin C. Suppression of normal lymphocyte function could be affected when splenic suppressor cells were separated from the normal population by a cell-impermeable membrane. Suppression occurred when the two populations were separated by a 0.8-µ Nuclepore membrane or by regular dialysis tubing but not when low porosity dialysis buting (m.w. <3,500 retained) was used. The suppressor cell in this system was nonadherent to plastic, partially adherent to nylon-wool, not removed by carbonyl iron and magnet, and pelletted to the bottom of a Hypaque-Ficoll gradient.