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Previous studies have shown that thymectomized lethally irradiated bone marrow grafted mice, reconstituted with thymocytes and pretreated with a large dose of sheep red blood cells (SRBC), are unable to respond to a subsequent immunizing injection of SRBC even after an inoculation of normal thymocytes. If, however, the mice are not thymocyte reconstituted prior to the pretreatment with SRBC, they can respond almost normally to an immunizing injection of SRBC if inoculated with normal thymocytes after the termination of antigen pretreatment. In the present study the immunosuppressive effect of the presence of thymocytes during the antigen pretreatrnent was studied by adoptively transferring the spleen cells of the antigen pretreated mice to thymus-deprived chimeras. These spleen cells not only did not co-operate with normal thymocytes in the secondary hosts, but they also prevented the co-operation of normal thymocytes with normal bone marrow derived cells. Untreated spleen cells or treated spleen cells from mice not reconstituted with thymocytes did not affect cell co-operation in the secondary hosts. The abrogation of the co-operation in the secondary host was specific in that the addition of spleen cells did not affect the anti-horse red blood cell response, if the primary host made antibody as a result of the pretreatment, the transfer of their spleen cells did not prevent antibody production in the secondary host. [ABSTRACT FROM AUTHOR]