Tumor cells in a cytostatic state caused by macrophages and antibody were isolated. Such suppressed cells excluded vital dye, incorporated uridine and leucine, and metabolized glucose. They did not, however, incorporate thymidine, nor did they resume cell division in culture. During prolonged culture, these cells eventually died. In this system, cytostasis was an all-or-nothing phenomenon at the level of the individual cell. Once in the cytostatic state tumor cells did not resume proliferation.