An interferon‐gamma ELISPOT assay has been developed for assessment of cellular immune responses to Varicella‐Zoster Virus (VZV) in large, multi‐center clinical vaccine trials. We show that the assay performed best when testing peripheral blood mononuclear cells (PBMCs) that had been isolated and then frozen on the same day as blood was drawn, and that freezing PBMCs from blood that was stored overnight before processing resulted in dramatically reduced responses. This assay was used to monitor cell‐mediated immunity (CMI) in response to a booster immunization with an investigational live, attenuated VZV vaccine in an elderly population that had been vaccinated 8–10 years previously. The booster vaccine elicited a 1.6‐ to 1.7‐fold rise in the VZV‐specific cellular immune response as measured by the ELISPOT assay. The increase from pre to post booster vaccination response was more pronounced (∼ 2.2‐fold rise) in a subset of subjects who had received two prior immunizations with a live, attenuated vaccine. J. Med. Virol. 70:S38–S41, 2003. © 2003 Wiley‐Liss, Inc.