To examine gender differences in immune reactions to stress and relationships between immune and cardiovascular reactivity, measures of cellular and mucosal immunity and cardiovascular activity were recorded in 77 men and 78 women at rest and in response to active (mental arithmetic) and passive (cold pressor) stress tasks. Both tasks reduced CD4+ T cells and the CD4/8 ratio. Total lymphocytes, NK cells, CD8+ T cells, and secretory immunoglobulin A (sIgA) increased with active stress. Passive stress decreased sIgA. At rest, men had more NK cells, less CD4+ T cells, and fewer neutrophils than women. Mental stress increased sIgA in men but not women. Cardiovascular reactivity to active stress was associated with increases in NK cells. The data support the hypothesis that stress-related increases in lymphocytes are beta-adrenergically mediated, and suggest that the fall in CD4+ T cells may be alpha-adrenergically driven. Mechanisms underlying sIgA reactions are more difficult to determine. Men and women differed in some cell counts, but not in reactivity, although gender influenced sIgA reactions to arithmetic.