Enriched populations of T or B cells, prepared by nylon wool filtration of mesenteric node lymphocytes from mice infected with Trichinella spiralis, were capable of transferring immunity to normal and irradiated syngeneic mice. In cell recipients there was an early loss of fecundity by the worms and an accelerated expulsion from the intestine. Treatment with anti-Thy 1.2 serum, to deplete contaminating T cells, severely reduced or abolished the protective activity of enriched B-cell fractions. Replacement of contaminating T cells by normal T cells restored the capacity of B-cell fractions to reduce worm fecundity but did not result in worm expulsion. As it was shown that comparatively small numbers of T cells (3 x 10(6)) alone were effective in transferring immunity to irradiated mice it is suggested that the T cells act not as helper cells, but are involved in the generation of changes in the intestinal environment that are detrimental to worm survival.