Classical conditioning studies in animals have demonstrated that humoral immunity can be conditioned. Human studies have shown that antihypertensive responses, anticipatory nausea, caffeine effects, and gastric activity can be conditioned, however, conditioning of the human immune system has never been demonstrated. This protocol is designed to test whether healthy human subjects can be conditioned to a stimulus that will activate their immune system. Specifically, an unconditioned stimulus, gamma interferon at a dose of established immunostimulation, is paired with a condition stimulus, propylene glycol. Through the four weeks of the protocol, subjects have repeated immune function testing, as the unconditioned stimulus is systematically withdrawn. Control groups consist of subjects receiving the unconditioned stimulus (propylene glycol) with placebo or the conditioned stimulus (gamma interferon) alone. Thus far, ten subjects have been accrued to the study. Four have withdrawn because of mild but disruptive toxicity. No laboratory results are available at this time.