Social support is supposed to have a positive health effect via alteration of immunity. In this study, associations between perceived social support and immune systems were examined. Immunological assessments, e.g. T cell count, Natural Killer cell count, Interferon-gamma, Interleukin-4, and psychological assessments, e.g. Generic Job Stress Questionnaire were conducted on male employees. Two-way (social support x job stressor) analyses of covariance controlling for age, smoking, alcohol consumption, and exercise revealed that there were main effects of perceived social support on NK cell counts, IL-4, and Th1/Th2 balance. On the other hand, interaction effects were observed on T cell counts and INF-gamma production in vitro. Social support affects immune function in a way that is consistent with both the direct and buffering hypotheses depending on the sources of support and the immune parameter.