Epstein-Barr virus (EBV)-specific cellular memory is not transferred from mother to child. Therefore, EBV-induced B-cell proliferation in in vitro-infected cord blood mononuclear cell cultures is not inhibited. However, by addition of immunomodulators, polysaccharide K (PSK) or truncated thioredoxin (Trx80) that activate monocytes, EBV-specific T-cell response could be generated in such cultures. Presently, we demonstrate that leukotriene B(4) (LTB(4)) is involved in the effect of the immunomodulators. LTB(4) was detected in the medium, and T-cell activation was compromised by addition of leukotriene biosynthesis inhibitors. Moreover, we found that LTB(4) added to infected cultures, which did not receive the immunomodulators, induced functional activation of the T cells. LTB(4) activated the monocytes and acted directly on the T cells. In consequence, addition of LTB(4) inhibited the EBV-induced proliferation of B lymphocytes. Specific cytotoxicity could be generated by restimulation of the T cells. The experiments showed successive stages of T-cell activation in acquisition of their immunologic effector function. This is orchestrated by complex cellular interactions, and autocrine loops mediated by soluble factors-here interferon (IFN)-gamma, interleukin (IL)-15, IL-12, and LTB(4). Importantly, the results indicate that endogenous LTB(4) can induce T-cell activation that inhibits the EBV-induced proliferation of B lymphocytes.