Immune serum accelerated the expulsion of Trichuris muris when transferred into normal mice on days 0 and 3 after infection, but had no effect when the recipient mice had been immunosuppressed by sublethal irradiation or by cortisone treatment. Delaying serum transfer until days 7 and 8 in normal mice failed to accelerate expulsion, although immune mesenteric lymph node cells (MLNC) accelerated expulsion whether transferred early or late in infection. Expulsion from NIH mice, normally complete by 12 days, was prevented by sublethal irradiation given as late as 9 days after infection, but could be restored by subsequent transfer of immune MLNC or, to a lesser degree, non-immune MLNC. Immune MLNC were unable to restore worm expulsion in mice irradiated before infection. These results are interpreted as showing that the immune expulsion of T. murius from mice during a primary infection requires the sequential activities of antibody-mediated and lymphoid cell-mediated components.