The immunogenicity and reactogenicity of live and heat-inactivated varicella vaccine were evaluated in 95 healthy seropositive adults (mean age, 32 yrs). Live-attenuated vaccine containing 28,000 pfu, 2800 pfu, or 280 pfu of Oka/Merck strain virus (8.4, 0.84, and 0.08 antigen units, respectively) or heat-inactivated vaccine with comparable antigen content (7.1, 0.71, and 0.07 antigen units) was administered subcutaneously to 15 to 16 adults per group in a randomized, single-blind study. ELISA antibody responses were dose-dependent but independent of whether the vaccine was live or inactivated. Mean titers reached a peak on day 14 and remained elevated through day 42 in recipients of the highest dosages but declined by followup at eleven months. A minority of vaccinees developed a four-fold or greater increase in antibody titer on day 14 (25 to 31 percent in the high-dose groups [greater than or equal to 7.1 antigen units], less than or equal to 7 percent of other groups). The vaccine was generally well tolerated. Localized erythema and swelling occurred at the injection site in 44 to 56 percent of the high- and middle dose recipients [greater than or equal to 0.71 antigen units], compared with 0 to 6 percent of those receiving the lowest dose. Although statistically significant increases in varicella antibody titer were observed after immunization with high doses of live or inactivated vaccine, the duration and clinical significance of this booster effect remains to be determined.