Since Thy1+ lymphokine-activated killer (LAK) cells have been implicated in protection against infection with vaccinia virus[18], [19], [20], [21], [22], and O'Leary, et al., have shown that the multiple compound-specific DTH responses they observed are mediated by a liver-resident Thy1+ NK cell population[1], we hypothesized that Thy1+ NK cells may be the effector population providing protection against secondary vaccinia virus infection. The gene discussed is THY1; the disease is infection.