The effects of examination stress on some lymphocyte subpopulations and cellular immune responses are reported. Twelve undergraduate students of psychology in examination term were tested six weeks before the written examination (phase I), one day before the first or second examination day (phase II) and 12-14 days after the examination (phase III). A comparable control group of students not in examination was assessed in parallel in phase II. The percentage of circulating monocytes increased in phase II in the examination group whereas the percentage of large (probably activated) CD4 and CD8 cells decreased. There was also a decrease in the number of cells expressing the IL-2 receptor in phase II. The proliferative response of T-cells to antigens, mitogens and allogeneic cells decreased from phase I to phase III. Thus, acute examination stress has a detectable influence on certain cellular immunological functions.