SummaryThe uptake of 3H‐thymidine in cultures of lymphocytes obtained from blood and lymphoid organs of sheep undergoing infection with Haemonchus contortus was used to assay the lymphocytes reactive to antigens and mitogens. In vitro reactivity to extracts from larvae of H. contortus or Trichostrongylus colubriformis was present in blood lymphocyte populations from worm‐free sheep. This indicated that worm‐free sheep possess lymphocytes with the capacity to respond to nematode parasite antigens and suggested that the success of infections by H. contortus may not depend upon a deficiency in antigen recognition by the host. In vitro reactivity to larval extracts, however. was virtually absent from the blood and lymphoid organs of sheep during active infections with H. contortus and appeared with greater intensity when infections had either declined naturally or had been removed by treatment with the anthelmintic, thiabendazole. Blood lymphocyte populations from infected sheep gave unimpaired in vitro responses to the phytomitogens, phytohaemagglutinin and concanavalin A, which may activate thymus‐derived cells, and to the hapten‐protein conjugate, dinitrophenyl‐bovine serum albumin, with which sheep were immunised. This indicated that infection by H. contortus did not cause a general immunosuppression either as a result of a polyclonal impairment of the transforming ability of lymphocytes or a reduction in the proportion of thymus‐derived cells among the lymphocytes present in blood. Infection with H. contortus may have produced an inactivation or deletion of antigen‐reactive lymphocytes which was selective for those cells which respond to parasite antigens. It is suggested that the induction of selective immunological unresponsiveness may be an adaptation enabling H. contortus to evade the immunological reactions of the sheep, thereby promoting the longevity of infections by this parasite.