Radiation chimaeras, prepared from NIH (rapid-responder) mice or from the F1 progeny of a cross between H-2 compatible B10.G (slow-responder) and NIH mice, were tested for their ability to respond to infection with the intestinal nematode parasite Trichinella spiralis. Mice reconstituted with bone marrow (BM) from NIH donors showed the rapid response characteristic of this strain, i.e. expelled worms from the intestine before day 12 of infection; those given BM from B10.G mice showed a show expulsion pattern, losing worms after day 12. There was no evidence that the environment of the recipient exerted any influence on the ability of the BM cells to express the response characteristic of the donor. When chimaeras were given immune mesenteric lymph node cells (IMLNC) from infected NIH donors there was successful adoptive transfer of immunity, resulting in an accelerated loss of worms. As before, the time course of the accelerated response was determined by the genotype of the BM used. These results confirm that genetic control of the process of worm expulsion is expressed at the level of a bone marrow-derived cell population and is independent of lymphocyte responsiveness. They further show that the factors involved are an inherent property of the cells concerned. The possibility that these cells are myeloid in nature is discussed.