Mesenteric lymph node cells (MLNC) prepared from BALB/c mice during infection with Hymenolepis nana proliferated extensively when cultured in the presence of soluble egg antigen, as assessed by measuring 3H-thymidine incorporation. Analysis of Hymenolepis-specific proliferative cells in MLNC by using monoclonal antibody specific for mouse T lymphocyte surface antigens revealed that the proliferative response of MLNC was mediated by Thy-1.2+, L3T4+ cells, that is, helper T cells. Supernatant of MLNC cultured with egg antigen contained large amounts of interleukin-2 and interferon-gamma, but only low levels of interleukin-5. The titer of these cytokines did not correlate with the interval between oral infection and collection of MLNC. These results strongly indicate that the Th1 subtype of helper T lymphocytes respond well to stimulation of H. nana egg antigen and suggest that acute inflammatory responses are involved in host-protective immunity to H. nana.