The cellular immune response of normal persons and marrow transplant recipients to varicella-zoster virus (VZV) antigen was measured with use of the lymphocyte transformation response. Ninety-nine of 100 normal persons with previous VZV infection had a stimulation index of greater than or equal to 4.8, while 10 susceptible persons had responses of less than or equal to 3.0. Before transplant, responses were lower than normal in patients with leukemia in relapse (P = 0.001), but not in patients with leukemia in remission or with aplastic anemia. Throughout the first 100 days after transplant, lymphocyte response was depressed (P less than 0.0005), especially among recipients of antithymocyte globulin during days 41-80 (P less than 0.05). Patients with aplastic anemia had higher responses than those with leukemia during days 20-60 (P less than 0.02). By one year, most responses were normal. Long-term survivors who had recurrent VZV infection had positive responses more often than those without recurrent infection (P = 0.01). The lymphocyte response to VZV antigen parelleled, and thus may predict, periods of increased susceptibility to VZV infection.