A local antibody response occurred in the small intestine when immune sheep were challenged with the enteric nematode parasite, Trichostrongylus colubriformis. Incoming infective larvae stimulated the appearance of increased numbers of IgA-containing lymphocytes in intestinal lymph. There were increases in the populations of IgA- and IgG1-containing plasma cells in the lamina propria. This did not occur in regions of the small intestine distant to the habitat of T. colubriformis. Challenge infection in immune animals increased the titre of anti-T. colubriformis haemagglutinating antibody in serum, intestinal lymph, intestinal fluid and extracts of intestinal mucosa. Although immune intestinal lymph contains anti-T. colubriformis antibody of IgA isotype, which is presumably produced in intestinal lamina propria, it failed to transfer passive immunity against the parasite. This result, however, does not exclude the possibility that IgA-antibody responses are important in protective immunity against T. colubriformis.