In the first preliminary experiment, mice were sensitized by 2 injections of an antigenadjuvant mixture and then infected with T. spiralis larvae. At this point, the experimentals were injected daily for 10 days with antithymocyte serum (ATS). This treatment failed to inhibit completely the immunity produced by the sensitizations. Although these mice harbored significantly fewer worms at 11 days after infection than the nonsensitized, untreated controls, they harbored significantly more worms than the sensitized, untreated controls. The nonsensitized, untreated controls harbored more worms than similar controls injected with normal rabbit serum, but the difference was not significant. A different effect of ATS was shown in the second preliminary experiment. In this case, 1 group of sensitized mice was injected with ATS 4 days before infection and daily thereafter until they were necropsied for counts of adult worms. They harbored about the same number of worms as the nonsensitized, untreated controls, whereas the sensitized controls not injected with ATS harbored significantly fewer. It is postulated that this demonstrated difference in the effect of ATS in the 2 experiments was due to the timing of the initial injection. In the final experiment, sensitized donors injected with ATS (experimentals) harbored about the same number of worms as the nonsensitized, untreated mice (regular controls), and significantly fewer were recovered from the sensitized, untreated mice (sensitized controls). Therefore, as in the second preliminary experiment, the ATS in this case inhibited completely the immunity established by the prior sensitizations. Spleen cells from the regular controls failed to confer immunity upon their recipients, but those from the experimentals and the sensitized controls produced a significant reduction in the number of adult worms recovered 11 days after the challenging infection. However, the latter harbored significantly fewer worms than the former, hence their immunity was considerably stronger. The results of this final experiment are discussed in light of reported effects of antilymphocyte and antithymocyte serum injections. Previous reports of this series (Larsh et al., 1964a, b, 1966, 1969, 1970a, b) included results interpreted by us to be in support of the hypothesis that the immunity of mice against the adult worms of Trichinella spiralis has a cell-mediated basis. That is, a specific delayed hypersensitivity (DH) is produced in response to an allergen within, or released by, the mature muscle larvae used for infection(s) or artificial sensitization. Upon challenge with living larvae, the DH response is thought to cause allergic inflammation due to injury of cells and tissues of the small intestine with the result that biochemical changes occur to produce unfavorable conditions that lead to worm expulsion. According to this concept, the specific DH reaction is the primary, but nonetheless indirect, event, whereas the resultReceived for publication 15 June 1972. * Supported in part by Grant AI-10,671 from the NIAID. ant inflammatory changes in the tissues are the secondary, but direct, event that leads to the elimination of significant numbers of worms. Despite the strong evidence in support of this hypothesis, conclusive proof of it awaits the demonstration that none of the many humoral factors known to be present in infected hosts plays a role in this particular aspect of the total immunity. Proof to satisfy the most severe critics might be difficult to obtain. For example, under certain conditions, pools of lymph node cells from guinea pig donors had the capacity to produce both DH and an accelerated humoral response in recipients (Kim et al., 1967), hence the results of conventional transfer studies might not provide the final answer. In any event, further work is needed in attempts to define precisely the roles of cell-mediated and humoral responses in causing the expulsion of adult worms. The present report deals with one such attempt, viz., the use of antithymocyte serum