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The cytotoxic reaction of effector cells on target cells was performed on cover slips which were set in a petri dish so as to be interposed from the dish surface through the bases of the slide glasses. After incubation, the target cells remaining on the cover slips were pulse-labeled with 3H-TdR for 45 min. Subsequently, the cover slips were removed from the bases, and dipped in cold 5% TCA to be freed from non-incorporated isotopes. The residual target cells adhering on the cover slips were counted without being removed.By this assay system, the specific or non-specific cytotoxicity of lymphocytes or macrophages was easily demonstrated with reliability. -cell-mediated cytotoxic assay; 3H -thymidine; cover slip; lymphocyte; macrophage

The DNA synthesis in syngeneie, allogeneic and xenogeneic cells was inhibited by peritoneal macrophages of normal WKA rats. This inhibitory effect of macro phages was found to be mediated by a soluble factor (MfID) released into a culture medium from macrophages which was heat-stable and non-dialyzable. The MfID was also responsible for the inhibition of lymphocyte DNA synthesis by MIX reaction.The action of MfID was indicated to suppress rather than to kill cell growth.It was interpreted that the inhibitory effect of macrophages on cellular DNA synthesis is their inherent property and macrophages may play the role of regulating the proliferation of lymphocytes and other cells. rat peritoneal macrophage; macrophage soluble factor; DNA synthesisIt is well known that macrophages are essential cells at the site of inflamma tion, wound repair, graft rejection, etc. However, the role of macrophages in these areas has not been well defined. Recently, macrophages have been shown to inhibit DNA synthesis in lymphocytes (Harris

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