Jackfruit lectin (JFL) from Artocarpus heterophyllus has been found to exhibit inhibitory activity in vitro with a cytopathic effect towards herpes simplex virus type 2 (HSV-2), varicella-zoster virus (VZV), and cytomegalovirus (CMV). The 50% inhibitory dose values from plaque reduction assay (inactivation) were 2.5, 5, and 10 Eg/ml of JFL for HSV-2, VZV, and CMV, respectively. Lymphocyte proliferation was significantly increased in the presence of the JFL in the concentration range of 2.5 to 50 Eg/ml, but was reduced at 500 Eg/ml. It was found that CD16(+)/CD56(+) cells (natural killer cells) were induced among the primary lymphocyte subpopulations. The activity of natural killer (NK) cells was not affected by JFL in the concentration range of 5 to 500 Eg/ml. These data suggest that JFL is mitogenic for NK lymphocyte (CD16(+)/CD56(+)) and also active against HSV-2, VZV, and CMV.