Experiments have been performed to examine the immune status in animals carrying tumors which are in their progressive phase of growth. Unfractionated spleen or lymph node cells of BALB/c mice, inoculated 10 to 14 days previously with murine sarcoma virus (MSV), have no demonstrable immunity in vitro to MSV-viral antigens. On fractionation of the spleen cells from such animals, however, large and small cells can be detected which react in well defined assays for immunity in this system. A cell population of intermediate size is capable of suppressing this activity. Treatment of the suppressing cell population with anti-θ or anti-immunoglobulin antiserum has provided data to suggest that the suppression is caused by a cell with immunoglobulin moieties on its surface. Moreover, evidence is presented to show that this suppression is nonspecific, in that the induction of protein synthesis in thymus-associated (T) cells in response to the mitogen phytohemagglutinin (PHA) is also suppressed by a cell with the above characteristics. The significance of these findings in relation to the status of growth of the tumor in the host is discussed.