Acute nutritional deprivation occurs frequently in clinical practice, yet little data exist on its effect on immune host defenses. To investigate this question, various immune parameters were studied in 15 obese subjects before and after a 14-day fast. Blood monocyte bactericidal activity and natural killer cell cytolytic activity were enhanced by fasting: monocyte killing increased in 12 of 14 subjects (p less than 0.05) and natural killer cell activity increased an average of 24 percent in 13 subjects tested (p less than 0.02). Starvation also enhanced parameters of humoral immunity as evidenced by increases in serum concentrations of IgG, IgA, and IgM (p less than 0.01). By contrast, lymphocyte blastogenic responses to the mitogen phytohemagglutinin were modestly decreased. Peripheral blood leukocyte counts, including neutrophils, T cells, and B cells, did not decrease significantly. These results indicate that fasting has differential influences on immune function rather than a uniformly deleterious effect. Of potential import, this nutritional alteration appears to actually enhance certain effector functions of the host defense system.