Young rats were not able to expel adult N. brasiliensis infections even when the worms were damaged by antibodies and the young rats were given all the cellular components (sensitized lymphocytes and bone marrow cells) shown to be necessary for the expulsion of antibody-damaged worms from adult rats. In contrast, most of the worms were expelled from young rats given sensitized lymph node cells on the day of a larval infection. These results show that the reduced ability of young rats to respond to infection by producing sensitized lymphocytes only partly explains their inability to expel the worms. It was not possible to explain the failure of young rats to expel adult worms by hypothesizing that they develop an active factor which prevents the cells from acting on the worms. It is also unlikely that worms persist in young rats because they differ in their susceptibility to cells compared with antibody-damaged worms from mature rats. This work suggests that the immune mechanism which affects the immature stages of this nematode may differ from that which controls the adult stages.