A study of immunoglobulin and albumin concentrations and the flow of intestinal lymph was made in sheep during infection with the parasitic nematode, Trichostrongylus colubriformis. Observations were made on uninfected worm-free sheep, on worm-free sheep undergoing primary infection and on immune sheep undergoing challenge infection. The flow of intestinal lymph and the concentrations of IgG1, IgG2 and IgM in intestinal lymph and IgG2 and IgM in blood plasma remained relatively constant during both primary and challenge infection. The concentration of IgA in intestinal lymph and blood plasma increased as early as day 6 of primary infection and maximum values were observed at day 34. By day 34 of primary infection the concentration of IgG1 in blood plasma was significantly greater than that found in worm-free controls. Plasma IgG1 concentrations in resistant sheep were also significantly greater than those observed in worm-free controls. Challenge infection in immune sheep did not raise the concentration of IgA in intestinal lymph and blood or IgG1 in blood. An indirect haemagglutination test was used to detect antibodies to T. colubriformis. Primary infection did not increase anti-worm antibody titres in either intestinal lymph or serum. Significant antibody titres were present in serum and lymph from immune sheep and were rapidly increased by challenge infection. Most antibody activity in serum was associated with IgM, whereas that in intestinal lymph was associated with IgA. These observations indicate the occurrence of a local antibody response to T. colubriformis in the intestine of immune sheep.