At times indicated, blood was obtained and processed as described in Materials and methods. Flow cytometric analysis was used to calculate numbers of naïve (CD28+, CD95−), memory (CD28+, CD95+) and effector (CD28−, CD95+) T cells (a and c) and their percentages of CD4 T cells (b and d). Total CD4 T cell counts increased significantly in naïve, memory and effector populations 14 days post-infection (dpi) in all monkeys compared to those before infection (a and c; p<0.05). After immunosuppression, all T cell subsets declined (a, p<0.05), whereas at 176 dpi (2 weeks before zoster), all CD4 T cells increased (a, p<0.05). The percentage of naïve, memory and effector CD4 T cells remained constant (b). CD4 T cells in the non-immunosuppressed monkey also declined after transportation to the radiation facility, but not to the extent seen in immunosuppressed monkeys (c); the percentages of naïve, memory and effector T cells also remained stable (d). Arrow indicates day of inoculation; black bar indicates time of immunosuppression. Data points represent the mean value +/− SEM.