Cumulating evidence suggests that immune parameters can be modified by behavioral conditioning processes in animals. The present results suggest that this also holds true for a human immune parameter. Healthy subjects were exposed to a conditioning procedure in which a neutral sherbet sweet (conditioned stimulus) was repeatedly paired with a subcutaneous injection of 0.2 mg epinephrine (unconditioned stimulus). After epinephrine administration an increase of natural killer (NK) cell activity could be observed (unconditioned response). On the conditioning test day the conditioned group showed increased NK cell activity after reexposure of the sherbet sweet combined with saline injection. No increase was found in control groups that previously received the sherbet sweet in combination with saline (saline control) or with epinephrine in an unpaired manner (unpaired control). This study supports previous findings of conditioned modulation of immune responses and represents a model to investigate conditioning processes of a human immune function.