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Beginning in 1967, our laboratory has published a number of investigations of the nonsyngeneic stem ceil-inactivation phenomenon (1-3). It has been shown that, after transplantation of a mixture of allogeneic lymphoid and hemopoietic cells into lethally irradiated hosts, stem elements of the mixture are inactivated by allogenic lymphocytes of the graft. In other words, after transplantation of such a cell mixture exogenous colony-forming units (CFU) ~ give no colonies in recipient spleens. On the basis of this fact we supposed that lymphoid cells of parental genot)~pe, being transplanted into sublethally irradiated F1 recipients, must inactivate host endogenous CFU, i.e., inhibit endogenous colony formation. The degree of inhibition may serve as a quantitatively accurate value for graft-versus-host (GVH) reaction. It is evident that such an experimental system is characterized by the development of two simultaneously occurring processes: multiplication of endogenous CFU and homograft reaction against them by transplanted lymphocytes. Both processes may be quantitatively estimated by counting the number of spleen colonies in animals of corresponding experimental groups. By this technique, it is possible to compare quantitatively two of the most important actions of immunosuppressive agents: their mitostatic effect (CFU inhibition), and their lymphotoxic action; the latter is a true immunosuppressive effect, the abolition of endogenous CFU inactivation. Materials and MethodsMice (20--22 g) of CBA, C57BL/6J inbred strains and (CBA X C57BL/6J)F1 were used in the experiments. Lymph nodes were removed from CBA and C57BL/6J mice and prepared cell suspensions (2) were injected into (CBA X C57BL/6J)F1 mice in amounts varying from 0.2 to 12 X l0 s cells. 24 hr before cell transfer, recipients were irradiated with 600 R gamma rays at a dose rate of about 438 R/min. For determination of endogenous CFU number, recipients were sacrificed 9-10 days after irradiation (4).Immunosuppressaslts at various dose ranges were injected intraperitoneally into (CBA X C57BL/6J)F1 mice. The first injection of 6-MP, Imuran, or anti-lymphocyte serum (ALS) was given simultaneously with cell transfer; cyclophosphamide was administered 1-2 hr 1 Abbreviations used in this paper: ALS, anti-lymphocyte serum; CFU, colony-forming units; GVH, graft-versus-host.

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1. CBA and C57B1 mice were exposed to LD50/30 doses of γ‐radiation and at intervals after irradiation the numbers of haemopoietic colony‐forming units (CFU) and of antibody‐producing cells in the spleen were determined. 2. CFU repression was observed after transfer of non‐irradiated cells to hybrid recipients. However, this repression did not appear until 60 days after irradiation of the donor. 3. The recovery after irradiation of the capacity to inactivate CFUs in spleen cell mixtures was studied. The results suggested that cells which produce 19S antibody are different from those which are reactive against foreign cells.