A CD30 ligand (CD30L, CD153) is a type II membrane-associated glycoprotein belonging to the TNF family. To illustrate the potential role of CD30L in CD4+ Th1 cell responses, we investigated the fate of Ag-specific CD4+ T cells in CD30L-deficient (CD30L−/−) mice after Mycobacterium bovis bacillus Calmette-Guérin (BCG) infection. The number of bacteria was significantly higher in organs of CD30L−/− mice than in wild-type (WT) mice 4 wk postinfection. The numbers of purified protein derivative-or Ag85B-specific-IFN-γ-producing-CD4+ T cells in spleen, lung, or peritoneal exudate cells were significantly fewer in CD30L−/− mice than in WT mice. During the infection, CD30L was expressed mainly by CD44+ CD3+ CD4+ T cells but not by CD3+ CD8+ T cells, B cells, dendritic cells, or macrophages. Costimulation with agonistic anti-CD30 mAb or coculturing with CD30L-transfected P815 cells restored IFN-γ production by CD4+ T cells from BCG-infected CD30L−/− mice. Coculturing with CD30L+/+ CD4+ T cells from BCG-infected WT mice also restored the number of IFN-γ+ CD30L−/− CD4+ T cells. When transferred into the CD30L+/+ mice, Ag-specific donor CD30L−/− CD4+ T cells capable of producing IFN-γ were restored to the compared level seen in CD30L+/+ CD4+ T cells on day 10 after BCG infection. When naive CD30L+/+ T cells were transferred into CD30L−/− mice, IFN-γ-producing-CD4+ Th1 cells of donor origin were normally generated following BCG infection, and IFN-γ-producing-CD30L−/− CD4+ Th1 cells of host origin were partly restored. These results suggest that CD30L/CD30 signaling executed by CD30+ T-CD30L+ T cell interaction partly play a critical role in augmentation of Th1 response capable of producing IFN-γ against BCG infection.