Human blood lymphocytes were stimulated with varicella zoster virus (VZV) antigen in limiting dilution cultures and the amounts of interferon-gamma (IFN-gamma) and IL-4 measured in the supernatants. The results indicate that up to 85% of proliferating cells of young adults produce IFN-gamma and up to 10% make IL-4. At limiting dilution, few if any wells were positive for both IFN-gamma and IL-4. The amount of IFN-gamma per well increased in the presence of antibody to IL-4, but anti-IFN-gamma not increase IL-4 production. The frequency of wells containing IFN-gamma was lower in subjects < 19 or > 55 years of age, and the amounts of IFN-gamma in positive wells was significantly lower in cultures of the older subjects' lymphocytes. The frequency of IL-4-making cells did not fall significantly with age. The data suggest that the age-related decline in the frequency of blood T cells which responds to VZV affects mainly the cells with a Th1-like cytokine phenotype.