Lymphocyte proliferative responses were evaluated in mucosal (mesenteric lymph nodes) and systemic (spleen and blood) lymphoid tissues of conventional piglets inoculated with the virulent or attenuated isolates of porcine epidemic diarrhoea virus (PEDV) strain CV-777 and challenged 21 days later with the virulent isolate of the same virus. A lymphoproliferative assay was developed in which mononuclear cells isolated from lymphoid tissues at different postinoculation and postchallenge days underwent a secondary in vitro stimulation with semipurified antigen obtained from PEDV-infected cell cultures. Vigorous lymphocyte proliferative responses were detected in the pigs inoculated with the virulent PEDV at postinoculation days 4-21, especially in the mesenteric lymph nodes and the blood; however, in the spleen this response was lower and less regular. The pigs inoculated with the attenuated virus showed a less intense response, the higher lymphocyte proliferation also corresponded to the mononuclear cells from mesenteric lymph nodes. Lymphocyte proliferation responses showed high correlations with protection against homologous challenge with virulent PEDV, and this correlation was higher in the gut associated lymphoid tissues (mesenteric lymph nodes). The cell proliferation response detected in blood mirrored that detected in the mesenteric lymph nodes, and showed also good correlation with protection. The results confirm that T-cell-helper function, assessed by lymphocyte proliferation responses, contributes to establishing a protective immune response against PEDV infections.