This study examined the effects of a 30-min laboratory stressor on aspects of immune function in 24 men and whether behavioral control over the stressor moderates stress effects. The stressor consisted of mild (2.5 mA) electric shock and loud (100 dB) white noise administered in an unpredictable, intermittent fashion. During stress sessions, only half of the subjects were able to control the stressor. Subjects with control were yoked to subjects who could not control the stressor so that both groups were exposed to identical intensity and duration of noise and shock. Immunologic function was assessed across stress and nonstress conditions by measuring changes in lymphocyte proliferation to concanavalin A (Con A) and phytohemagglutinin (PHA) and by measuring changes in percentages of lymphocytes and their subpopulations, granulocytes, and monocytes. Results revealed that exposure to the uncontrollable stressor altered mood but did not affect immune function. In contrast, exposure to controllable stress did not alter mood but did result in lowered lymphocyte proliferation to Con A. Poststress percentages of monocytes were also lower in subjects exposed to the controllable stressor. Results suggest that acute stress can alter aspects of immune function in humans and underscore the importance of stressor controllability in moderating stress effects on human immunity.