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The determination of biological activities of anti-idiotypic (Id) antibodies is a useful approach for studying antigen-specific T cell recognition unit because cross-reactive Id determinants have been shown to be shared by antibodies and T cells (1, 2). AntiId antibodies were reported to trigger Id-specific T cells that suppress antibody formation (3) and delayed-type hypersensitivity (DTH) responses (4). Helper and mixed-lymphocytic-reactive T cells were also reported to be induced by anti-Id antibodies (2, 5). Recently, we have reported that anti-Id serum produced in C57BL/ 6 mice against C3H.SW anti-poly(LTyr,LGlu)-poly(DLAia)--poly(LLys) [(T,G)-A--L] antibodies stimulated in vitro proliferative responses of (T,G)-A--L-primed T cells (6). Furthermore, anti-Id sera against (T,G)-A--L-specific antibodies (7) reacted with (T,G)-A--L-specific helper factors produced by educated T cells (8), a T cell-specific hybrid line, and a (T,G)-A--L-specific continuous line with helper activity (9). D T H responses to (T,G)-A--L are T cell mediated, antigen specific (10), and genetically controlled (11). In a previous article we have described the participation of two distinct T cell subsets in D T H to (T,G)-A--L (12). We have shown that sensitized radioresistant Lyt-1+2-3 cells required the presence of normal radiosensitive Lyt-l+2+3 + cells for efficient D T H responses. It was of interest to establish the effect of murine anti-Id serum against (T,G)-A--L-specific antibodies on T cellmediated D T H responses. In this report we describe the ability of this anti-Id serum to replace the antigenic challenge in the efferent phase of DTH. We were able to localize the effect of the antiserum on the antigen-educated Lyt-1+2-3 cells.