SummaryImmunity to the adult stage of Trichinella spiralis, assessed by an acceleration of worm expulsion, was transferred to recipient mice with mesenteric lymph node cells (MLNC) or serum taken from infected donors. Immunity was transferred most effectively by MLNC taken from donors infected for 8 days, i.e. donors actively responding to infection. Transfer of both MLNC and serum brought about a marked acceleration of worm expulsion in all cases, even where MLNC or serum given separately failed to transfer a significant degree of immunity.