Reserpine treatment inhibited the expulsion of the intestinal nematode parasite Trichostrongylus colubriformis by adoptively immunized guinea pigs, but similar treatment of immune cell donors did not inhibit the capacity of their mesenteric lymph node cells to transfer immunity against the parasite. The treatment schedule used in these experiments decreased the levels of both histamine and 5-hydroxytryptamine in the small intestinal mucosa. Utilizing the rejection of transplanted fourth-stage larvae from the small intestine of immune guinea pigs as a model of worm expulsion, it was shown that a single injection of reserpine inhibited worm expulsion and caused a reduction in the level of 5-hydroxytryptamine, but not of histamine in the small intestinal mucosa. The results support the hypothesis that 5-hydroxytryptamine is involved in the expulsion of T. colubriformis by immune guinea pigs.