The immunosuppressive effect of normal RNA has been reported previously by several authors (1–4), but its site of action is unknown. Recently, we (5) found that normal RNA depressed the immunologic system when injected 5 days before the antigen, confirming the results of others (1, 2). These experiments show that normal RNA effects both humoral and cellular immunity. The purpose of this work is to determine where in the immunologic sequence RNA acts when administered before the antigen. If the

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... mmunologic sequence is assumed to be: antigen-macrophage-sensitized immunocyte (lymphocyte or plasma cell), it should be possible to restore the normal immunologic response after the administration of RNA by injecting these cells from isogenic normal donors if RNA acts on them. In the present experiment the action of normal macrophages has been investigated.