Natural cytostatic cells have a broader range of antitumor activity than natural killer cells.

Natural cytotoxic activity by Thy-1 negative, asialo-GM-1 positive and column-nonadherent spleen cells was detected against YAC-1 target cells but not against Meth-A and EL-4 target cells as reported by many investigators. In the present study, on the other hand, natural cytostatic activity was detected in Thy-1 negative, asialo-GM-1 negative and column-nonadherent cells and this activity was effective against YAC-1, Meth-A and EL-4 target cells. Such a cytostatic activity was not modified by the functional levels of T cells in the donor mice of effector cells. The cytostatic activity against Meth-A and EL-4 target cells was detected not only in an allogeneic system but also in a syngeneic system. The cytostatic activity was not depressed in a tumor-bearing state in contrast to the NK activity. The cytostatic activity as well as the NK activity was detected in spleen and peritoneal cells but not in lymph node cells of the various sources. The spleen and peritoneal cavity may play a role distinct from that of lymph nodes in the resistance against tumor development.